[ARM Documentation] Clarify -mcpu, -mtune, -march

gcc/

	PR target/59718
	* doc/invoke.texi (-march=): Clarify documentation for ARM.
	(-mtune=): Likewise.
	(-mcpu=): Likewise.




git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@207501 138bc75d-0d04-0410-961f-82ee72b054a4