Fix Texinfo usage issues. From Patrice Dumas <pertusus@free.fr>.

Fix Texinfo usage issues.  From Patrice Dumas <pertusus@free.fr>.

	* gdb.texinfo (Summary): Add a link to "Free Documentation".
	(Free Documentation): Add @node line.
	(Continuing and Stepping) <advance location>: Use @item instead of
	@itemx.
	(Reverse Execution) <set exec-direction reverse>: Use @item instead of
	@itemx.
	(Embedded Processors): Put the link to "PA" to its correct place,
	according to document structure.
	(Python API): Put the link to "Lazy Strings In Python" to its
	correct place.
	(Index): Use @@ in @tex block to get a literal @.

	* gdbint.texinfo: Fix the @subtitle line.

	* stabs.texinfo (Top): Put the link to "Symbol Types Index" to its
	correct place.