document the new manifest restrictions on name & path settings

Bug: https://crbug.com/gerrit/14156
Change-Id: I473edab1173e6a266d0754c29d5dc7ff761f1359
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/299403


Reviewed-by: Michael Mortensen <mmortensen@google.com>
Tested-by: Mike Frysinger <vapier@google.com>