DOC: Addition to pd.DataFrame.assign() docstring (#20085)
- [ ] closes #20085 
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Adding information to pd.DataFrame.assign() docstring that re-assigning of existing columns will overwrite the same.

cc @jreback  # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20702?src=pr&el=h1) Report
> Merging [#20702](https://codecov.io/gh/pandas-dev/pandas/pull/20702?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d5d5a718254f45b4bdc386c360c830df395ec02a?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20702/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20702?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #20702   +/-   ##
=======================================
  Coverage   91.84%   91.84%           
=======================================
  Files         153      153           
  Lines       49277    49277           
=======================================
  Hits        45257    45257           
  Misses       4020     4020
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.23% <100%> (ø)` | :arrow_up: |
| #single | `41.91% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20702?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/20702/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.16% <100%> (ø)` | :arrow_up: |

------

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> **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/20702?src=pr&el=footer). Last update [d5d5a71...6d43c4a](https://codecov.io/gh/pandas-dev/pandas/pull/20702?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @HannahFerch 