DOC: Fix versionadded for cond in .where
Per discussion in <a href="https://github.com/pandas-dev/pandas/pull/15414#discussion_r103089692">#15414 (comment) </a>.

@jorisvandenbossche 
The 'other' keyword below that has the same issue. > The 'other' keyword below that has the same issue.

Done. @gfyoung Thanks!