gas: doc: update documentation for the new listing option

Add a new listing option, -i, to emit ginsn in the listing output.  We
may also emit other SCFI information if necessary in the future.

ginsn are most useful when seen alongside the assembly instructions.
Hence, they are emitted when the user includes the assembly instructions
in the listing output, i.e., "-ali=FILE".

gas/doc/:
	* as.texi: Add documentation for the new listing option, -i.