QHash::insert: Remove outdated \note

It was added in 5.15 when QHash could have multiple entries per key.

Pick-to: 6.2 6.3
Change-Id: Ifb855b05dd65bbc67bb64bf2567229394525b2aa
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/tools/qhash.cpp

@@ -2175,9 +2175,6 @@ size_t qHash(long double key, size_t seed) noexcept
 
     If a key is common to both hashes, its value will be replaced with the
     value stored in \a other.
-
-    \note If \a other contains multiple entries with the same key then the
-    final value of the key is undefined.
 */
 
 /*! \fn template <class Key, class T> bool QHash<Key, T>::empty() const