Improve "set scheduler-locking" documentation

Improve "set scheduler-locking" documentation

This improves the "set scheduler-locking" documentation in the GDB
manual:

 - Use a table to describe the four available modes.

 - Describe "step" in terms of "on" and "off".

 - Tweak the "replay" mode's description to describe replay first
   instead of recording, and also mention how the mode behaves during
   normal execution.

 - Say what is the default mode.

Change-Id: Ie12140138b37534b7fc1d904da34f0f174aa11ce