DOC: Add documentation for freq='infer' option of DatetimeIndex and TimedeltaIndex constructors
- [X] closes #21128
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

Reboot of #21201.  Should be good to merge on green based on comments in the previous PR, but will leave this open for a little bit to give anyone who missed the old PR a chance to review.
lgtm. thanks @jschendel 