DOC: missing apt.rst links

[jreback]

might be nice to adapt this scipy script: https://github.com/scipy/scipy/blob/master/tools/refguide_check.py

• To find all things that are not documented
• make an issue so they can get fixed.
• add this procedure to the wiki

[jorisvandenbossche]

There is already done some work on this:

• Discussion here: Add indexer and str to API documentation #5124 (comment), with a notebook doing something similar. I had an adapted version of this notebook somewhere, but I should look that up.
• A PR implementing something like this: ENH: added api_rst_coverage.py #8166, which should be reviewed (I will try to do that)

So we should consolidate the different efforts, and maybe see if we can use some things of the scipy version.

[jreback]

ahh forgot about that issue
can u put an xref on that link and ping seth-p?