Document LDAP lock group owners

Problem to solve

We have a new feature for 13.0 per gitlab-com/www-gitlab-com!49384 (closed)

Current docs at https://docs.gitlab.com/ee/administration/auth/ldap-ee.html#global-group-memberships-lock are not adequate.

Further details

Proposal

• Add info to https://docs.gitlab.com/ee/administration/auth/ldap-ee.html#global-group-memberships-lock on how to enable / disable "Lock memberships to LDAP synchronization"
• Add info to appropriate APIs on how to set up lock_memberships_to_ldap and unlock_memberships_to_ldap

Who can address the issue

Other links/references