DOC: Add notes about M and Y to to_timedelata documentation.  (#34968)
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Not sure it closes these issues, but using "M" in the arg argument of to_timedelta caused confusion in #34968 and #27285.  #33094 is somewhat related. I came across another issue I can't find now where there was confusion about "Y" returning a days with times delta