Fix documentation about document lock

Paragraph Locking the document says " A lock can be removed by the owner of the document or by someone having the manage permission"

=> it's incorrect: the Manage permission does not allow to unlock a document. Only administrators and the locker can unlock.

Same mistake in https://doc.nuxeo.com/userdoc/content-lock/

Note that I've created NXP-27710 to add lock details in the document summary, so we won't have to fix this other error in the documentation.