doc: RISC-V: Document the `-misa-spec=' option

doc: RISC-V: Document the `-misa-spec=' option

We have recently updated the default for the `-misa-spec=' option, yet
we still have not documented it nor its `--with-isa-spec=' counterpart
in the GCC manuals.  Fix that.

	gcc/
	* doc/install.texi (Configuration): Document `--with-isa-spec='
	RISC-V option.
	* doc/invoke.texi (Option Summary): List `-misa-spec=' RISC-V
	option.
	(RISC-V Options): Document it.