[IMP] doc: explain db-filter case

The %d %h options in db-filter are case sensitive but the URL are not.
If you happen to use databases with uppercase in the name, using (?i) may be
needed to match the correct database.

Fixes #17407

doc/reference/cmdline.rst

@@ -159,7 +159,11 @@ database
     - ``%h`` is replaced by the whole hostname the request is made on.
     - ``%d`` is replaced by the subdomain the request is made on, with the
       exception of ``www`` (so domain ``odoo.com`` and ``www.odoo.com`` both
-      match the database ``odoo``)
+      match the database ``odoo``).
+
+      These operations are case sensitive. Add option ``(?i)`` to match all
+      databases (so domain ``odoo.com`` using ``(?i)%d`` matches the database
+      ``Odoo``).
 
 .. option:: --db-template <template>