docs: mention incompatibility between kernel oplocks and streams_xattr

docs: mention incompatibility between kernel oplocks and streams_xattr

See BUG https://bugzilla.samba.org/show_bug.cgi?id=7537 for details.

Signed-off-by: Bjoern Jacke <bj@sernet.de>
Autobuild-User(master): Björn Jacke <bj@sernet.de>
Autobuild-Date(master): Fri Sep 12 18:29:37 CEST 2014 on sn-devel-104

(cherry picked from commit 27b2ddcddea5c32a84016b5c6843eb37d2798adf)

Bug: https://bugzilla.samba.org/show_bug.cgi?id=10814

Autobuild-User(v4-0-test): Karolin Seeger <kseeger@samba.org>
Autobuild-Date(v4-0-test): Thu Sep 18 11:15:53 CEST 2014 on sn-devel-104