Note that session() should not be used

According to requests/api.rst, the `Session` class was originally named session in 0.x. [When the class name was capitalized in 1.x](https://github.com/requests/requests/blob/265ef609d5903151374fba480aa81aafe68126ff/docs/api.rst#migrating-to-1x), the `session()` method was added for backwards compatibility.

This adds a docstring note that `requests.Session` should be used instead

# [Codecov](https://codecov.io/gh/requests/requests/pull/4685?src=pr&el=h1) Report
> Merging [#4685](https://codecov.io/gh/requests/requests/pull/4685?src=pr&el=desc) into [master](https://codecov.io/gh/requests/requests/commit/883caaf145fbe93bd0d208a6b864de9146087312?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/requests/requests/pull/4685/graphs/tree.svg?height=150&width=650&token=mZGswIiXLa&src=pr)](https://codecov.io/gh/requests/requests/pull/4685?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #4685   +/-   ##
=======================================
  Coverage   66.79%   66.79%           
=======================================
  Files          15       15           
  Lines        1563     1563           
=======================================
  Hits         1044     1044           
  Misses        519      519
```


| [Impacted Files](https://codecov.io/gh/requests/requests/pull/4685?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [requests/sessions.py](https://codecov.io/gh/requests/requests/pull/4685/diff?src=pr&el=tree#diff-cmVxdWVzdHMvc2Vzc2lvbnMucHk=) | `76.27% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/requests/requests/pull/4685?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/requests/requests/pull/4685?src=pr&el=footer). Last update [883caaf...1c13c5e](https://codecov.io/gh/requests/requests/pull/4685?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).


@nateprewitt In that case, I'll add a [`DepreciationWarning`](https://docs.python.org/3/library/exceptions.html#DeprecationWarning) telling users to use `Session` instead.

I tried to track down when `Session` was added, but it seems to have been [sometime before `v0.8.4`](https://github.com/requests/requests/blob/v0.8.4/requests/sessions.py#L50), the oldest tagged version on GitHub.

On closer inspection, it seems `session` was created one day *after* `Session`. ([Aug 16, 2011](https://github.com/requests/requests/commit/519b9cee969e7364654a05ed466db253d35370d3) vs [Aug 15, 2011](https://github.com/requests/requests/commit/ca428504d485b0495b5e794cdd1b47bbb0bd890f))

@nateprewitt I've reverted the docstring changes in favor of a `DepreciationWarning`.

I'm not sure this is worth the grief we'll receive for adding this DeprecationWarning.

I’m in agreement with @sigmavirus24. The DeprecationWarning will likely be more of a harassment for existing users than deterrent for new users.

I think the initial scope of a minor docstring change is likely the extent of what we’d want in this PR.

Noted. I've removed the depreciation warning and updated the docstring.

Should I squash these commits together?

@Stevoisiak, please.

@nateprewitt Squashed and rebased against master.

This looks good to me, thanks @Stevoisiak!