Document when config options were deprecated

When going through the code and looking for what code was deprecated and
when, its easier to know if there is an explicit comment. Otherwise one
has to trace through 'git log' and 'git blame' to be 100% sure that its
ok to remove the code.

Change-Id: I57b5861e57168c44729934757e732de962f3d91a
Merge "Document when config options were deprecated"