(MIPS): Improve documentation of heuristic-fence-post. Document

	(MIPS): Improve documentation of heuristic-fence-post.  Document
	"set/show mips abi", "set/show mips saved-gpreg-size", "set/show
	mips stack-arg-size", "set/show mips mask-address", "set/show mips
	remote-mips64-transfers-32bit-regs", "set/show debug mips".
	(ARM): Document ARM-specific commands.
	(AVR): New section.  Document "info io_registers".
	(CRIS): New section.  Document "set/show cris-version" and
	"set/show cris-dwarf2-cfi".
	(HPPA): New section.  Document "set/show debug hppa" and "maint
	print unwind".
	(Netrino): New subsection.  Document "set/show debug nto-debug".
	(Super-H): New section.  Document the "regs" command.