[docs] Add missing qHash(Enum) documentation

Amends fb4bc5fa262336504e0f28603658bb2572796ce5.

Pick-to: 6.5
Change-Id: I1097b8313806c5f9272bcbfced21e66036e45763
Reviewed-by: Mate Barany <mate.barany@qt.io>
Reviewed-by: Juha Vuolle <juha.vuolle@qt.io>
(cherry picked from commit 55a6c77d77dab4d877e1c4bb9d9b828b005604eb)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/tools/qhash.cpp

@@ -1369,6 +1369,17 @@ size_t qHash(long double key, size_t seed) noexcept
     }
 }
 
+/*!
+    \fn template <typename Enum, std::enable_if_t<std::is_enum_v<Enum>, bool> = true> size_t qHash(Enum key, size_t seed)
+    \since 6.5
+    \qhashbuiltin
+
+    \note Prior to Qt 6.5, unscoped enums relied on the integer overloads of this
+    function due to implicit conversion to their underlying integer types.
+    For scoped enums, you had to implement an overload yourself. This is still the
+    backwards-compatible fix to remain compatible with older Qt versions.
+*/
+
 /*! \fn size_t qHash(const QChar key, size_t seed = 0)
     \since 5.0
     \qhashold{QHash}