[meta.reflection.extract] Reinstate "Constant When" wording from P2996R13 regarding similar and function pointer types

I'm not sure if the mention of function pointer types was dropped
in a misguided editorial simplification, but this is not an editorial
change. (But we retain the cross-reference which was not in the paper.)

Author: tkoeppe

source/meta.tex

@@ -5949,8 +5949,9 @@
 \begin{itemize}
 \item
   \tcode{U} is a pointer type,
-  \tcode{T} and \tcode{U} are similar types\iref{conv.qual}, and
-  \tcode{is_convertible_v<\brk{}U, T>} is \tcode{true},
+  \tcode{T} and \tcode{U} are either similar\iref{conv.qual}
+  or both function pointer types, and
+  \tcode{is_convertible_v<U, T>} is \tcode{true},
 \item
   \tcode{U} is not a pointer type
   and the cv-unqualified types of \tcode{T} and \tcode{U} are the same,