Add informal definition of "label" in note
The term "label" is used extenisively, but (AFAICT) it is never explained what a "label" is. Is an integer offset a label? Do labels apply to rows and columns, multi-indexes? What's the relation between an "index" "axis" and "label" ... etc.

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/27005?src=pr&el=h1) Report
> Merging [#27005](https://codecov.io/gh/pandas-dev/pandas/pull/27005?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b4d4ec5a36acda40f13a8c3c3e19262c095d4c41?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/27005/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/27005?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #27005      +/-   ##
==========================================
- Coverage   91.99%   91.99%   -0.01%     
==========================================
  Files         180      180              
  Lines       50774    50774              
==========================================
- Hits        46712    46708       -4     
- Misses       4062     4066       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <ø> (ø)` | :arrow_up: |
| #single | `41.85% <ø> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/27005?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/27005/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `88.88% <0%> (-11.12%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/27005/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.89% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/27005?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/27005?src=pr&el=footer). Last update [b4d4ec5...d592cb3](https://codecov.io/gh/pandas-dev/pandas/pull/27005?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/27005?src=pr&el=h1) Report
> Merging [#27005](https://codecov.io/gh/pandas-dev/pandas/pull/27005?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b4d4ec5a36acda40f13a8c3c3e19262c095d4c41?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/27005/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/27005?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #27005      +/-   ##
==========================================
- Coverage   91.99%   91.99%   -0.01%     
==========================================
  Files         180      180              
  Lines       50774    50774              
==========================================
- Hits        46712    46708       -4     
- Misses       4062     4066       +4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <ø> (ø)` | :arrow_up: |
| #single | `41.85% <ø> (-0.09%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/27005?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/27005/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `88.88% <0%> (-11.12%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/27005/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `96.89% <0%> (-0.12%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/27005?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/27005?src=pr&el=footer). Last update [b4d4ec5...0374241](https://codecov.io/gh/pandas-dev/pandas/pull/27005?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I find this note a little confusing. Would just mentioning that a `label` is an object that provides an alternative to positional selection suffice? I agree. It is too verbose, and there is already too many notes and warnings (which I tend to have a habit of not reading anyway :)). I will re-word. I agree that we should be more explicit on the terms we used. I think we discussed in the past about having a glossary, which I think it's the right way to address this. I could only find this related issue: #21784

I don't think this PR is helping much. I'll close this, and open an issue for the glossary, feel free to work on that. Sounds good. May be loosely intertwined with what we are defining in pandas._typing

@sam-at-github thanks for the PR. To the point above though I think we'd rather make a more comprehensive change than what is offered here. Would certainly value input if you'd like to put thoughts into the other issue Sorry, re-reading my last comment, just realized that probably didn't sound really nice... Not my intention.

@sam-at-github thanks for your contribution, and it really helps, since this is very helpful feedback. I opened #27019 since I think a glossary will make it easier to address what you're addressing here. Feel free to continue the work you started here, but I think it makes more sense to close this, and continue in a PR that addresses that issue.

Thanks! Sounds like perfect is the enemy of the good around here. Good luck with glossary.