MIPS/GCC/doc: Fix `d' constraint description

MIPS/GCC/doc: Fix `d' constraint description

Revert a part of the `d' constraint documentation update made with
commit 111868 ("Use define_constraint for MIPS"),
<https://gcc.gnu.org/ml/gcc-patches/2006-03/msg00460.html>,
<https://gcc.gnu.org/ml/gcc-patches/2006-03/msg00541.html>, which
inadvertently made the registers covered to be referred to as "address"
rather than "general-purpose" registers, and then further clarify the
set of registers allowed with MIPS16 code.

	* config/mips/constraints.md (d): Fix documentation.
	* doc/md.texi (Machine Constraints): Update accordingly.

From-SVN: r240546