doc, m68k: Clean up M680x0 option documentation [PR122243] [PR119404]

doc, m68k: Clean up M680x0 option documentation [PR122243] [PR119404]

gcc/ChangeLog
	PR other/122243
	PR target/119404
	* config/m68k/m68k.opt (mlra): Fix typo in help string.
	* doc/invoke.texi (Option Summary) <M680x0 Options>: Remove
	redundant -mno- forms from the list.
	(M680x0 Options): Combine documentation for -mshort, mbitfield,
	-msep-data, -mid-shared-library with that for their respective
	negatives that were formerly separately listed.  Add missing
	@opindex entries.