docs: warn about password resets in ceph RGW doc

Keeping this value to true with external RGW as object storage endpoints
can cause outages that are annoying to recover from. We should warn
operators about this.

Change-Id: I37965382bed6b9762782ec3f9ec3103143d738cc
Signed-off-by: Bertrand Lanson <bertrand.lanson@infomaniak.com>
(cherry picked from commit 8510b76)

Author: ednxzu

doc/source/reference/storage/external-ceph-guide.rst

@@ -537,6 +537,15 @@ This prevents cross-project and public object access. This can be resolved by
 setting ``ceph_rgw_swift_account_in_url`` to ``true``. This should match the
 ``rgw_swift_account_in_url`` configuration option in Ceph RadosGW.
 
+.. warning::
+
+   When using an external Ceph RadosGW for object storage endpoints
+   (when ``enable_ceph_rgw: true``), operators are advised to set
+   ``update_keystone_service_user_passwords: false`` in``globals.yml``.
+   Leaving this option enabled causes the service user password to be reset
+   on every reconfigure, which invalidates existing tokens and can cause
+   unforeseen outages for object storage consumers.
+
 Load balancing
 ==============