doc: Add 2 missing return types to \fn commands

Return types were missing from two \fn commands, but clang-qdoc
did not detect they were missing on macOS. clang-qdoc on linux
did detect them. This could mean there is a problem with the
function that matches a \fn signature with its declaration in
the database. It might be too forgiving.

Change-Id: I01f107a6162e2f36b594b5d3dd4029b8ef2db678
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qglobal.cpp

@@ -3610,7 +3610,7 @@ bool qunsetenv(const char *varName)
 */
 
 /*!
-    \fn template <typename T> qAsConst(T &t)
+    \fn template <typename T> typename std::add_const<T>::type &qAsConst(T &t)
     \relates <QtGlobal>
     \since 5.7
 
@@ -3662,7 +3662,7 @@ bool qunsetenv(const char *varName)
 */
 
 /*!
-    \fn template <typename T> qAsConst(const T &&t)
+    \fn template <typename T> void qAsConst(const T &&t)
     \relates <QtGlobal>
     \since 5.7
     \overload