DOC: Documentation for pandas.core.indexes.api
The functions on pandas.core.indexes.api had almost no documentation. This is an attempt to explain them better and a progress towards #22915.
Hello @araraonline! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/indexes/api.py`](https://github.com/araraonline/pandas/blob/76c3e5dd035b8de33e606a73347aa5b1258c9d12/pandas/core/indexes/api.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22980?src=pr&el=h1) Report
> Merging [#22980](https://codecov.io/gh/pandas-dev/pandas/pull/22980?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f6da1f1ec3b68a9210be158a693ac389076fa179?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22980/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22980?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #22980      +/-   ##
==========================================
+ Coverage   92.19%   92.19%   +<.01%     
==========================================
  Files         169      169              
  Lines       50911    50922      +11     
==========================================
+ Hits        46939    46950      +11     
  Misses       3972     3972
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.62% <ø> (ø)` | :arrow_up: |
| #single | `42.3% <ø> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22980?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/22980/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9hcGkucHk=) | `98.91% <ø> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/22980/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `96.65% <0%> (ø)` | :arrow_up: |
| [pandas/core/arrays/base.py](https://codecov.io/gh/pandas-dev/pandas/pull/22980/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2Jhc2UucHk=) | `95.89% <0%> (ø)` | :arrow_up: |
| [pandas/core/util/hashing.py](https://codecov.io/gh/pandas-dev/pandas/pull/22980/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdXRpbC9oYXNoaW5nLnB5) | `98.4% <0%> (+0.03%)` | :arrow_up: |
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/22980/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `93.44% <0%> (+0.12%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22980?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/22980?src=pr&el=footer). Last update [f6da1f1...62e7a3e](https://codecov.io/gh/pandas-dev/pandas/pull/22980?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 context looks ok, @datapythonista or @jorisvandenbossche if you want to glance at the doc-strings; these are internal only