DOC GH22897 Fix docstring of join in pandas/core/frame.py
- [X] closes #22897
- [X] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

#22900 was dealt with by adding an "import pandas as pd" line to the start of the examples section.
Hello @JustinZhengBC! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/frame.py`](https://github.com/JustinZhengBC/pandas/blob/cf5669862842c6a7042056f32d3e70c3ec4b7472/pandas/core/frame.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22904?src=pr&el=h1) Report
> Merging [#22904](https://codecov.io/gh/pandas-dev/pandas/pull/22904?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/14598c6f82973b33d8b55e1e493f6103f9e16a62?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22904/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22904?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22904   +/-   ##
=======================================
  Coverage   92.18%   92.18%           
=======================================
  Files         169      169           
  Lines       50830    50830           
=======================================
  Hits        46860    46860           
  Misses       3970     3970
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.6% <ø> (ø)` | :arrow_up: |
| #single | `42.37% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22904?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/22904/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.2% <ø> (ø)` | :arrow_up: |

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[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22904?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22904?src=pr&el=footer). Last update [14598c6...05988d4](https://codecov.io/gh/pandas-dev/pandas/pull/22904?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Alright, I believe I have taken care of all the other issues now. I fixed those issues. I also added some backticks around parameter names in other sections, as that was what other docstrings seemed to be doing. I changed the backticks around the df's to be single again. Superseded by #23471