Add documentation explaining how to read Commit and Tag

Since we stopped supporting these types for reading, but their
name is a natural candidate for someone to try and use in code,
explain where they should be looking instead.

Change-Id: I091a1b0ef71b842016020f938ba3161431aab9c9
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
2 files changed