doc, epiphany: Clean up epiphany target options and docs [PR122243]

doc, epiphany: Clean up epiphany target options and docs [PR122243]

gcc/ChangeLog
	PR other/122243
	* config/epiphany/epiphany.opt (mlong-calls): Make it do something
	useful.
	(may-round-for-trunc): Make this undocumented option with a weird
	name an alias for -mmay-round-for-trunc.
	(mfp-iarith): Fix doc string.
	* doc/invoke.texi (Option Summary) <Adapteva Epiphany Options>:
	Add missing options.
	(Adapteva Epiphany Options): Document negative forms also when
	that is not the default, or where it's unclear.  Document
	-may-round-for-trunc and -mfp-iarith.  Fix spelling of
	-mpost-inc and -mpost-modify.