[IMP] reference/cli: db-filter for Web UI

The odoo-bin help for the db-filter says:
`Regular expressions for filtering available databases for Web UI.`

However, the current documentation does not mention that the filters
only affect the Web UI, and not the cron workers.

closes #15375

Signed-off-by: Sylvain Francis (syf) <syf+odoo@odoo.com>

Author: iCart

content/developer/reference/cli.rst

@@ -239,7 +239,7 @@ Database
 
 .. option:: --db-filter <filter>
 
-    hides databases that do not match ``<filter>``. The filter is a
+    hides databases that do not match ``<filter>`` for the Web UI. The filter is a
     `regular expression`_, with the additions that:
 
     - ``%h`` is replaced by the whole hostname the request is made on.
@@ -289,6 +289,8 @@ Database
     If database 11seconddatabase doesn't exist, the database is created and base modules
     is installed
 
+    .. warning:: This option does not affect cron workers, if no --database is given, cron workers will run on every available database
+
 .. option:: --db-template <template>
 
     when creating new databases from the database-management screens, use the