Update docs: add "recommended" to the "optional" label on the headers

Signed-off-by: Suguru Hirahara <[email protected]>

Author: Suguru Hirahara

docs/configuring-playbook-dimension.md

@@ -14,7 +14,7 @@ See the project's [documentation](https://github.com/turt2live/matrix-dimension/
 
 Dimension can only be installed after Matrix services are installed and running. If you're just installing Matrix services for the first time, please continue with the [Configuration](configuring-playbook.md) / [Installation](installing.md) and come back here later.
 
-### Register a dedicated Matrix user (optional)
+### Register a dedicated Matrix user (optional, recommended)
 
 We recommend that you create a dedicated Matrix user for Dimension (`dimension` is a good username).
 

docs/configuring-playbook-email2matrix.md

@@ -22,7 +22,7 @@ If you really need to run an email server on the Matrix machine for other purpos
 
 For details about using Email2Matrix alongside [Postfix](http://www.postfix.org/), see [here](https://github.com/devture/email2matrix/blob/master/docs/setup_with_postfix.md).
 
-### Register a dedicated Matrix user (optional)
+### Register a dedicated Matrix user (optional, recommended)
 
 We recommend that you create a dedicated Matrix user for Email2Matrix.