gdb/doc: Consistency fixes for GDB/MI documentation

gdb/doc: Consistency fixes for GDB/MI documentation

I noticed two inconsistencies in the GDB/MI documentation, which this
commit addresses:

  1. Each MI command is introduced like this:

     @subheading The @code{-command-name} Command

     Except for a few of the tracing command, which just use:

     @subheading -command-name

     In this commit I've updated all these trace commands to use the
     more common format.

  2. Each MI command starts with a @subheading, and then the details
     of that command are split up using multiple @subsubheading
     entries.

     Except for a few commands which use @subheading for the top-level
     command, and then continue to use @subheading for each part of
     the command description.

     In this commit I've updated these to use @subsubheading where
     appropriate.