[docs] Adjust qExchange() docs for the present, where C++17 is required [1/2]: 6.2

In C++17, std::exchange() is unconditionally available, so focus
attention on the missing constexpr.

Pick-to: 6.4 6.3 6.2
Change-Id: Ia09bf6da7bd62e5a41083cfc5253e30a0298099f
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qttypetraits.qdoc

@@ -64,8 +64,9 @@
     Replaces the value of \a obj with \a newValue and returns the old value of \a obj.
 
     This is Qt's implementation of std::exchange(). It differs from std::exchange()
-    only in that it is \c constexpr already in C++14, and available on all supported
-    compilers.
+    only in that it is \c constexpr already before C++20.
+
+    We strongly advise to use std::exchange() when you don't need the C++20 variant.
 
     Here is how to use qExchange() to implement move constructors:
     \code