DOC: Removed references to v0.17 in docs and doc strings
- [ x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

This PR removes references to changes and "versionadded" for pandas v0.17. This can be considered a continuation to #17504.

I propose pulling this in as soon as 0.21 is released.

Alternatively, these could be accepted in v0.21, as these references to v0.17 IMO are too old to be useful to the average doc reader (approx. 2 years) and upgraders could just reference the 0.20 docs or the whatsnew sections of v0.21.