Document why ReplicationUser doesn't use registered groups

I forgot why we did this isEmpty test before adding the registered
groups into the set.  Explain what the heck is going on.

Change-Id: Ic62ec8b56b139aa3e9276fc9f3c9b2d3da12273c
Signed-off-by: Shawn O. Pearce <sop@google.com>
1 file changed