documentation: Document Read Access +2 aka Upload Access

We failed to document this feature, even though we added it way
back in 2.0.24.  Document it now.

Change-Id: I5088a1b13fca0ef3d57d55c1be5e71420c5e64ec
Signed-off-by: Shawn O. Pearce <sop@google.com>
1 file changed