Document 'distclean-stage[N]'

Refer to 'Makefile.tpl':

    # Rules to wipe a stage and all the following ones, also used for cleanstrap
    [+ IF prev +]distclean-stage[+prev+]:: distclean-stage[+id+] [+ ENDIF prev +]
    .PHONY: distclean-stage[+id+]
    distclean-stage[+id+]::
    	@: $(MAKE); $(stage)
    	@test "`cat stage_last`" != stage[+id+] || rm -f stage_last
    	rm -rf stage[+id+]-* [+
    	  IF compare-target +][+compare-target+] [+ ENDIF compare-target +]

	gcc/
	* doc/makefile.texi (Makefile Targets): Document
	'distclean-stage[N]'.