Manual:$wgRestrictionLevels

Details
Set of permission keys that can be selected on the 'page protection' page. 'autoconfirmed' allows all registered users if $wgAutoConfirmAge and $wgAutoConfirmCount are 0.

If you add additional levels (e.g. 'docwriters') those can be named via editing the content of 'MediaWiki:Protect-level-docwriters'.

Don't forget to adjust your group permissions accordingly; e.g.: If you set a level higher than sysops, that is, protection from sysop editing, sysops cannot give a page that level of protection nor remove it, even with the 'protect' permission.

A user with the 'protect' permission still cannot protect a page that he/she can't edit, such as if defined as such by $wgNamespaceProtection.