(Object Storage) S3 performance gain from disabling verify_bucket_exists

As developed in nextcloud/server#23932

Also includes caveat language re: multibucket scenarios

Signed-off-by: Josh Richards <josh.t.richards@gmail.com>

admin_manual/configuration_files/primary_storage.rst

@@ -196,7 +196,7 @@ Optional parameters less commonly needing adjustment:
 * :code:`putSizeLimit` defaults to :code:`104857600`
 * :code:`legacy_auth` has no default
 * :code:`version` defaults to :code:`latest`
-* :code:`verify_bucket_exists` defaults to :code:`true`
+* :code:`verify_bucket_exists` defaults to :code:`true` [Note: Setting this to :code:`false` *after* confirming the bucket has been created may provide a performance benefit, but may not be possible in multibucket scenarios.]
 
 **If you are using Amazon S3:** the :code:`region` parameter is required unless you're happy with 
 the default of :code:`eu-west-1`. There is no need to override the :code:`hostname` or :code:`port`.