DEPR: camelCase in offsets, get_offset
- [x] closes #4205
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

@datapythonista the good news is im able to reproduce this locally with `make doc` like you suggested, the bad is that I have no idea why I'm getting a bunch of these complaints:

```
/home/vsts/work/1/s/doc/source/reference/api/pandas.tseries.offsets.BQuarterBegin.isAnchored.rst: WARNING: document isn't included in any toctree
```

ideas?
 We fixed a lot of these in the past, but I'm not sure if we ever understood the problem. At least I don't think I did.

I think what we do is to add the docs being reported to a hidden autosummary. See: https://github.com/pandas-dev/pandas/blame/master/doc/source/reference/index.rst#L44

@jorisvandenbossche probably knows more > BQuarterBegin.isAnchored.rst: WARNING: document isn't included in any toctree

It's because you removed those from the api doc pages, while they are still generated automatically from the class docstring. But so they also need to be included somewhere in another autosummary that adds them into the toctree. So like Marc said, we use hidden autosummaries to solve this. There should be some other examples in the api docs of that (look at the bottom of those pages). See the "hidden" text in `doc/source/reference/index.rst` (https://github.com/pandas-dev/pandas/blob/master/doc/source/reference/index.rst) thank you both @jorisvandenbossche my attempts at mimicing the examples in offset_frequency.rst haven't worked.  in this commit i used "api.offsets.Foo.onOffset" but itt also didn't work with "Foo.onOffset", "api/Foo.onOffset", "pandas.tseries.offsets.Foo.onOffset" The toctree needs the path (which becomes the url, eg https://dev.pandas.io/docs/reference/api/pandas.tseries.offsets.DateOffset.isAnchored.html), so you need something like `api/pandas.tseries.offsets.DateOffset.isAnchored`. However, if you do it on this page, it might need to be `../api/pandas...` since it is one level down in the hierarchy. @jorisvandenbossche see newest commit where i tried just about every variant I could think of, including putting them in a different file where we already use this pattern. >  is there any difference between get_offset and to_offset that the user should be aware of?

I dont think get_offset is supported in a meaningful enough way for this to make sense.  e.g. i just found the example in the docstring was wrong > did you add a deprecation note in whatsnew?

yes great thanks.