Man, had to really dig to find that. Ctrl-F "lock" didn't hit anything on the landing page or main docs page.
Even then it looks like an implementation that misses the mark. Both Perforce and SVN mark "lockable" files as Read-Only on a sync of the local copy. It looks like all this does is prevent a push of the file which will be the same result as if someone merged it ahead of you.
Without that guard in place it makes it really easy to make a mistake and then have changes to a binary file that will collide. Having a checkout process as part of locking workflow is pretty much mandatory.
[edit]
Nevermind, I finally found this[1] that explains Git LFS does it correctly, kudos! Could do with some more detailed documentation since it looks partially implemented from the man pages.