[dcl.constexpr] Add "consteval" to subclause heading (#2597)

Author: gamecentric

source/declarations.tex

@@ -735,8 +735,9 @@
 The \tcode{friend} specifier is used to specify access to class members;
 see~\ref{class.friend}.
 
-\rSec2[dcl.constexpr]{The \tcode{constexpr} specifier}%
+\rSec2[dcl.constexpr]{The \tcode{constexpr} and \tcode{consteval} specifiers}%
 \indextext{specifier!\idxcode{constexpr}}
+\indextext{specifier!\idxcode{consteval}}
 
 \pnum
 The \tcode{constexpr} specifier shall be applied only to