Mediawiki:growthexperiments-edit-config-mentorship-description currently says:

Wikis can have two separate mentor lists: one for mentors that get randomly assigned to all newcomers, and another for mentors that do not get randomly assigned, but can claim mentees. This second list is used for mentors that only want to be mentors to people from events or classes.

The two lists are now gone, as the configuration is now handled at Special:ManageMentors

Also, following T321056: Turn mentorship off on all wikis where the community has not yet setup a mentorship list, we need to make sure that communities are informed about the ideal number of people needed to turn the feature on, or in order to recruit more mentors.

New text proposal:

Mentors can have two roles. By default, they are randomly assigned to newcomers. An option exists for mentors that only want to be mentors to people from events or classes: they don't get any newcomers assigned to them, but they can claim them.
The ideal number of mentors is: one mentor for each group of 500 new accounts, with a minimum of 3 mentors.
[[ mw:Special:MyLanguage/Growth/FAQ#Mentorship | Know more about how mentorship works ]]