doc: Document that client_dirsize_rbytes confuses rsync

doc: Document that client_dirsize_rbytes confuses rsync

This is important to document because otherwise the immediate question
one has is "why _wouldn't_ I enable this?".
At the same time, being able to use tools like rsync is a common
motivation for using CephFS.

Unfortunately the only source to this I could find is the presentation
"CephFS: Architecture Introduction & New Features" by Greg Farnum:
https://ceph.io/assets/pdfs/events/2025/ceph-days-silicon-valley/10%20-%20Greg%20-%20CephFS.pdf

Signed-off-by: Niklas Hambüchen <mail@nh2.me>