Clarify that clean URLs in WordPress are for front end pages

https://docs.civicrm.org/sysadmin/en/latest/install/wordpress/#enabling-cleaner-urls-for-wordpress says

"Starting in Version 5.13.x CiviCRM now can support "Clean URLs" for WordPress"

It is probably worth clarifying that this is for front end pages. We could change it to

"Starting in Version 5.13.x CiviCRM now can support "Clean URLs" for WordPress for front end pages"