doc: show C++14 and C++20 constexpr as [constexpr] in the docs.

Ideally it'd say the standard version as well, but that requires adding
a feature to m.css. Not *that* important right now.

doc/Doxyfile

@@ -2262,7 +2262,8 @@ PREDEFINED             = DOXYGEN_GENERATING_OUTPUT \
     CORRADE_ENUMSET_OPERATORS(message)= CORRADE_UNUSED= \
     MAGNUM_BUILD_DEPRECATED MAGNUM_TARGET_GL MAGNUM_TARGET_VK \
     MAGNUM_GL_ABSTRACTSHADERPROGRAM_SUBCLASS_DRAW_IMPLEMENTATION(type)= \
-    MAGNUM_GL_ABSTRACTSHADERPROGRAM_SUBCLASS_DISPATCH_IMPLEMENTATION(type)=
+    MAGNUM_GL_ABSTRACTSHADERPROGRAM_SUBCLASS_DISPATCH_IMPLEMENTATION(type)= \
+    CORRADE_CONSTEXPR14=constexpr CORRADE_CONSTEXPR20=constexpr
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then this
 # tag can be used to specify a list of macro names that should be expanded. The