doc, c: Clarify status of decimal floating point support [PR123234]

At first glance, the GCC manual's section documenting decimal floating
point support as a C extension appears to be bit-rotten, since this
feature is now standardized in ISO C23.  In fact, though, GCC's
implementation is based on the older N1312 proposal, and full C23
support is not yet available.  This patch makes the GCC manual say that
explicitly.

gcc/ChangeLog
	PR c/123234
	* doc/extend.texi (Decimal Float): Clarify that GCC really does
	support only the older N1312 draft as documented here, not the
	standard C23 functionality yet.