gdb: Rewrite documentation for layout and focus commands.

gdb: Rewrite documentation for layout and focus commands.

Changes the documentation for the layout and focus commands.

Instead of documenting each layout (or focus) sub-command as a separate
command, document a single layout (and focus) command which takes a
parameter, then list the possible parameters in a table nested under
each command.

The documentation for the layout command has been extended little to
make it clearer which windows are shown in each layout.

gdb/ChangeLog:

	* doc/gdb.texinfo (TUI): Restructure documentation on TUI layout
	and focus commands.