fix: Fix session lock example and add more explanations

[happenslol]

The example would previously show a solid color for 1.5s, after which the lock surfaces were created. With multiple displays, the surfaces would sometimes not be created at all.

The issue here is that the compositor expects a surface to be created for each display before locked is called, and since the example only creates the surfaces in locked the compositor would time out waiting for the surfaces, resulting in the placeholder solid color screen.

I've also added some more comments, clarifying some things that I ran into while trying to use the protocol myself.

Lastly, I've changed the finished log, since from my understanding, finished is only called if the session failed to be locked, and will not be called if the session is locked and unlocked normally.