__nextcloud: add comments about the maintenance mode

... because it not always work like you want it.

type/__nextcloud/gencode-remote

@@ -101,6 +101,9 @@ REMOTE
 
 # Apply some misc to the installation folder.
 elif [ "$install" ]; then
+    # Maintainer mode is not available before installation
+
+    # Correct all file permissions of the new installation
     cat << REMOTE
 chown '$user':'$group' -R '$installdir'
 REMOTE

type/__nextcloud/man.rst

@@ -262,6 +262,16 @@ download it into the temp directory because it does not find a better location.
 In legacy, it was downloaded to the parent directory, but this may not the best
 location as the installation dir can be everywhere.
 
+This type does not garantee to always show the maintenance mode screen because
+nextcloud does not show it in every case:
+
+1.  For fresh installations, the maintenance mode can not be set.
+2.  While upgrades starting at version 20, the user is promted to execute the
+    update manually via the webinterface instead of the maintenance screen.
+
+It is recommended to show an own maintanance screen via the webserver if this
+is critical for you.
+
 
 SEE ALSO
 --------