BUG: support skew function for custom BaseIndexer rolling windows

[AlexKirko]

• xref BUG: rolling window functions don't support custom indexers #32865
• 1 tests added / 1 passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

Scope of PR

This PR does a couple things to fix the performance of skew, and this also fixes the behavior of all the functions that have require_min_periods for small values of min_periods:

• clarify docs. Name all sample methods uniformly, add a note to the caveat that users should in general be careful about using sample methods with windows
• fix bug in _apply that made us never go into the BaseIndexer control flow branch
• fix bug in _apply: pass window_indexer.window_size into calc_min_periods instead of min_periods or 1. We want the custom indexer window size there, so it's not clear to me why we were passing min_periods or 1 .

Details

The algorithm itself is robust, but it defaults to sample skewness, which is why there was a difference between its output and numpy. To prevent misunderstandings, I clarified the docs a bit.
We were also passing a wrong value to calc_min_periods, and we weren't going into the proper if branch, because we were checking the type of window instead of self.window.

Background on the wider issue

We currently don't support several rolling window functions when building a rolling window object using a custom class descended from pandas.api.indexers.Baseindexer. The implementations were written with backward-looking windows in mind, and this led to these functions breaking.
Currently, using these functions returns a NotImplemented error thanks to #33057, but ideally we want to update the implementations, so that they will work without a performance hit. This is what I aim to do over a series of PRs.

Perf notes

No changes to algorithms.

[AlexKirko]

@jreback @mroeschke
Here is the fix for skew with custom rolling windows. Mainly, this returns sample skewness, similarly to std and var, but I also stumbled onto a couple adjacent bugs while calling this with small values of min_periods.

Fixed those and edited the docs to be more clear.

Details in the description above.

[AlexKirko]

Did something happen? Why are we running CI only on macOS? Can't find the issue that led to this . . .

[jreback]

#33747

[jreback]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[AlexKirko]

@jreback
There still seem to be problems with CI. I'm reasonably confident that this fix isn't platform-dependent, but please say if you'd like to wait until we can launch all the pipelines and have them report back.

Plenty horrible stuff gets into software because 'it should have worked'.

[jreback]

k can you merge master again

CI should run

[AlexKirko]

@jreback
Done, all green.

[jreback]

thanks @AlexKirko