CWG2652 Overbroad definition of __STDCPP_BFLOAT16_T__

Also fixes NB US 31-071 (C++23 CD).

Author: burblebee

source/preprocessor.tex

@@ -1766,7 +1766,8 @@
 \mname{STDCPP_BFLOAT16_T}\\
 Defined as the integer literal \tcode{1}
 if and only if the implementation supports an extended floating-point type
-with the properties described in \ref{basic.extended.fp}.
+with the properties of the \grammarterm{typedef-name} \tcode{std::bfloat16_t}
+as described in \ref{basic.extended.fp}.
 
 \item
 \indextext{__time__@\mname{TIME}}%