Add documentation for lock_on_merge capability

What does this MR do and why?

Adds documentation for the lock on merge request feature, Add documentation for lock_on_merge capability (#422214 - closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #422214 (closed)