DOC: fix doc build warnings

[jorisvandenbossche]

Bunch of small fixes that caused warnings/errors

[jreback]

lgtm

as an aside not averse to having the doc build fail on (certain types of warnings)

e.g. PR adds docs which then cause more warnings

not sure how hard this would actually be though (maybe sphinx can do it?)

[jorisvandenbossche]

Yes, I know (and think there is an issue about that somewhere). The problem is that the current doc build is not yet fully warning-free (also not after this PR), of which some I don't directly know how to fix. So as long that is not the case, we cannot let it fail on that.

[jorisvandenbossche]

We could let sphinx fail on a a warning/error, or otherwise parse the log and if certain words are present (like 'ERROR') then let travis fail (and in this way, be a bit more flexible on what is not allowed to fail).

[codecov-io]

Codecov Report

Merging #15505 into master will decrease coverage by -0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15505      +/-   ##
==========================================
- Coverage   90.36%   90.36%   -0.01%     
==========================================
  Files         136      136              
  Lines       49553    49553              
==========================================
- Hits        44781    44780       -1     
- Misses       4772     4773       +1

Impacted Files	Coverage Δ
pandas/io/html.py	84.48% <ø> (ø)	✅
pandas/io/parsers.py	95.51% <ø> (ø)	✅
pandas/io/json/normalize.py	97.87% <ø> (ø)	✅
pandas/core/generic.py	96.31% <ø> (ø)	✅
pandas/core/common.py	91.02% <ø> (-0.34%)	❌

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