DOC : Update generic.py
Included documentation for 'how' in DataFrame.resample

- [x] closes #21463
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21465?src=pr&el=h1) Report
> Merging [#21465](https://codecov.io/gh/pandas-dev/pandas/pull/21465?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/576d5c6b76e039a411a7cc4c0de29813e2de0149?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/21465/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/21465?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21465   +/-   ##
=======================================
  Coverage    91.9%    91.9%           
=======================================
  Files         153      153           
  Lines       49606    49606           
=======================================
  Hits        45589    45589           
  Misses       4017     4017
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.3% <ø> (ø)` | :arrow_up: |
| #single | `41.89% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/21465?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/21465/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.12% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21465?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/21465?src=pr&el=footer). Last update [576d5c6...cd9954e](https://codecov.io/gh/pandas-dev/pandas/pull/21465?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 This was deprecated quite some time ago

https://github.com/pandas-dev/pandas/commit/14cf67fbb25c1695fd126ab034af2218d04c154c