Documentation: Updating docblocks in optimizers.py
## Summary
Updating documentation for the optimizer classes. I explicitly state that the scheduler classes are multiplying the optimizer's learning rate by a changing variable.

## Closes
https://github.com/huggingface/transformers/issues/1712
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1737?src=pr&el=h1) Report
> Merging [#1737](https://codecov.io/gh/huggingface/transformers/pull/1737?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/e99071f10578adb0191288c1f3301e9a758d6200?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/1737/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/transformers/pull/1737?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master    #1737   +/-   ##
=======================================
  Coverage   83.93%   83.93%           
=======================================
  Files          94       94           
  Lines       13951    13951           
=======================================
  Hits        11710    11710           
  Misses       2241     2241
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/1737?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [transformers/optimization.py](https://codecov.io/gh/huggingface/transformers/pull/1737/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL29wdGltaXphdGlvbi5weQ==) | `96.62% <ø> (ø)` | :arrow_up: |
| [transformers/tests/modeling\_common\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1737/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX2NvbW1vbl90ZXN0LnB5) | `73.08% <0%> (-0.59%)` | :arrow_down: |
| [transformers/tests/modeling\_tf\_common\_test.py](https://codecov.io/gh/huggingface/transformers/pull/1737/diff?src=pr&el=tree#diff-dHJhbnNmb3JtZXJzL3Rlc3RzL21vZGVsaW5nX3RmX2NvbW1vbl90ZXN0LnB5) | `96.8% <0%> (+1.59%)` | :arrow_up: |

------

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> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/1737?src=pr&el=footer). Last update [e99071f...c958962](https://codecov.io/gh/huggingface/transformers/pull/1737?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Great, thanks for this!