docs: add warnings about repos data model

For people coming across these docs and thinking that repo's methods
are good to replicate, add a note warning them against doing so.

Change-Id: I443a783794313851a6e7ba1c39baebac988bff9a
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/298164


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