MIPS/include: opcode/mips.h: Update stale comment for CODE20 operand

MIPS/include: opcode/mips.h: Update stale comment for CODE20 operand

Complement commit 1586d91e32ea ("/ 0 should send SIGFPE not SIGTRAP..."),
<https://sourceware.org/ml/binutils/2004-07/msg00260.html>, and update a
stale comment referring the 20-bit code field of the BREAK and SDBBP
instructions, by making it explicit that where permitted by choosing the
MIPS32 or a later ISA the whole field can now be set with a single
operand for the SDBBP instruction only.

	include/
	* opcode/mips.h: Update comment for MIPS32 CODE20 operand.