DOC: Add Timestamp, Period, Timedelta, and Interval to api.rst

[GuessWhoSamFoo]

• closes DOC: add Timestamp / Period / Interval to api.rst #17369
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #17424 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #17424      +/-   ##
==========================================
- Coverage   91.15%   91.14%   -0.02%     
==========================================
  Files         163      163              
  Lines       49581    49591      +10     
==========================================
  Hits        45198    45198              
- Misses       4383     4393      +10

Flag	Coverage Δ
#multiple	88.92% <ø> (ø)	⬆️
#single	40.25% <ø> (-0.07%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/io/parquet.py	65.38% <0%> (-0.37%)	⬇️
pandas/core/frame.py	97.72% <0%> (-0.1%)	⬇️
pandas/core/indexes/base.py	96.29% <0%> (ø)	⬆️
pandas/core/indexes/multi.py	96.9% <0%> (ø)	⬆️
pandas/plotting/_core.py	82.69% <0%> (+0.01%)	⬆️
pandas/core/indexes/range.py	92.28% <0%> (+0.09%)	⬆️
pandas/core/indexes/datetimes.py	95.43% <0%> (+0.09%)	⬆️

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[jreback]

Can you add properties and methods for these Scalars? (you might be able to get away with a directive to list all of them, for Period/Interval might be ok, Timestamp will show everything from its inheritied classes). If you can build the docs and show would be great (if there are too many things shown, we might just list some attributes / methods and point to the datetime.datetime docs otherwise.)

Can you also add for Timedelta.

[GuessWhoSamFoo]

Ends up looking like above for Timestamp and Period. There's also room to clarify days_in_month vs daysinmonth as a compatibility alias. It looks like a lot of the existing properties don't need to be there - any guidance on this?

[jreback]

also these need to go in a new section: Scalars I think (alternatively they could go with there associated Indexes), but I think a Scalars section is fine.

[jreback]

thanks!

note pls have a look at the built docs (may take a bit for it to generate) http://pandas-docs.github.io/pandas-docs-travis/

want to make sure that searching works ok.

Then of course we want to enhance doc-strings where appropriate (which is already an outstanding issue IIRC), if you are interested.

[jreback]

http://pandas-docs.github.io/pandas-docs-travis/api.html#scalars

couple of corrections needed

Timedelta.is_populated should be removed (that's really an internal property. we should renamed it actually to _is_populated)
Timestamp.from_ordinal is not spelled correctly (.fromordinal is the method)
Timdelta.to_pytimedelta misspelling
Timedelta.view, exclude this
Timedelta.delta exclude