doc/cephfs: use 'p' flag to set layouts or quotas

Remind users that clients must use the 'p' flag when writing file
layouts.

This PR is made in response to
https://www.reddit.com/r/ceph/comments/1gbad51/comment/ltkgd5v/

Co-authored-by: Zac Dover <[email protected]>
Signed-off-by: TruongSinh Tran-Nguyen <[email protected]>

Author: truongsinh

doc/cephfs/client-auth.rst

@@ -106,6 +106,8 @@ If quotas are not enabled or if no quota is set on the mounted sub-directory,
 then the overall usage of the file system will be reported irrespective of the
 value of this setting.
 
+.. _cephfs-layout-and-quota-restriction:
+
 Layout and Quota restriction (the 'p' flag)
 ===========================================
 

doc/cephfs/file-layouts.rst

@@ -6,6 +6,9 @@ File layouts
 The layout of a file controls how its contents are mapped to Ceph RADOS objects.  You can
 read and write a file's layout using *virtual extended attributes* or xattrs.
 
+Clients must use the ``p`` flag when writing a file's layout. See :ref:`Layout
+and Quota restriction (the 'p' flag) <cephfs-layout-and-quota-restriction>`.
+
 The name of the layout xattrs depends on whether a file is a regular file or a directory.  Regular
 files' layout xattrs are called ``ceph.file.layout``, whereas directories' layout xattrs are called
 ``ceph.dir.layout``.  Where subsequent examples refer to ``ceph.file.layout``, substitute ``dir`` as appropriate