AVR: Some minor improvements to the TEXI documentation.

gcc/
	* config/avr/avr-devices.cc (avr_texinfo): Adjust documentation for
	Reduced Tiny.
	* config/avr/gen-avr-mmcu-texi.cc (main): Add @anchor for each core.
	* doc/extend.texi (AVR Variable Attributes): Improve documentation
	of io, io_low and address attributes.
	* doc/invoke.texi (AVR Options): Add some anchors for external refs.
	* doc/avr-mmcu.texi: Rebuild.