DOC: clarify docs related to deprecation of indexing DataFrame with single partial datetime-string
A follow-up on https://github.com/pandas-dev/pandas/pull/36179#discussion_r494830867 (cc @jbrockmendel). Attempt to clarify what is actually deprecated, remove the `:okwarning:` for several cases in the docs that were not actually deprecated, and updated to use the preferred `.loc` for some others.