DOC: Surface / doc mangle_dupe_cols in read_excel
xref https://github.com/pandas-dev/pandas/issues/10523#issuecomment-438465073.

We already support `mangle_dupe_cols` (in the same vein as CSV).  It was just hidden in `**kwds`.  This just documents it in `read_excel` (it behaves exactly as it does in `read_csv`) and tests that we inform the end user that `mangle_dupe_cols=False` is not supported...yet.

Explicitly surfacing relevant CSV-related arguments will allow us to eventually remove the magical `**kwds` and obviate need for validation (xref #22189).

Hello @gfyoung! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/io/excel.py`](https://github.com/gfyoung/pandas/blob/59549404e8285c8fc44d777b215ae9cdab9c3339/pandas/io/excel.py) !

 - There are no PEP8 issues in the file [`pandas/tests/io/test_excel.py`](https://github.com/gfyoung/pandas/blob/59549404e8285c8fc44d777b215ae9cdab9c3339/pandas/tests/io/test_excel.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23678?src=pr&el=h1) Report
> Merging [#23678](https://codecov.io/gh/pandas-dev/pandas/pull/23678?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/a197837c4067469478b13e491a21b58cb78ed165?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/23678/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23678?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #23678   +/-   ##
=======================================
  Coverage   92.24%   92.24%           
=======================================
  Files         161      161           
  Lines       51318    51318           
=======================================
  Hits        47339    47339           
  Misses       3979     3979
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.63% <ø> (ø)` | :arrow_up: |
| #single | `42.31% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23678?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/23678?src=pr&el=footer). Last update [a197837...5954940](https://codecov.io/gh/pandas-dev/pandas/pull/23678?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @gfyoung 