docs: smart-sync: split out & expand details

The existing documentation on smart-sync behavior is a bit light on
details, and out of date wrt what the code actually does.  Start a
dedicated document and fill it out more.

Change-Id: I1a8a3ac6edf9291d72182ad55db865035d9b683e
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/450002


Commit-Queue: Mike Frysinger <vapier@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>