Docs: Add "Lock items" to work items permissions doc

The following discussion from !177498 (merged) should be addressed:

• @amandarueda started a discussion: (+3 comments)

  @msedlakjakubowski I don't see locking work items discussions in the permissions list. I see "Update merge request including assign, review, Code Suggestions, approve, labels, lock and resolve threads", WDYT about adding it as a generic entry (covering all work items)?