[Bug 49018][lldb] Fix incorrect help text for 'memory write' command

Certain commands like 'memory write', 'register read' etc all use
the OptionGroupFormat options but the help usage text for those
options is not customized to those commands.

One such example is:

  (lldb) help memory read
           -s <byte-size> ( --size <byte-size> )
               The size in bytes to use when displaying with the selected format.
  (lldb) help memory write
	   -s <byte-size> ( --size <byte-size> )
               The size in bytes to use when displaying with the selected format.

This patch allows such commands to overwrite the help text for the options
in the OptionGroupFormat group as needed and fixes help text of memory write.

llvm.org/pr49018.

Reviewed By: DavidSpickett

Differential Revision: https://reviews.llvm.org/D114448