Document QMultiHash::clear function

When QMultiHash derived from QHash, it inherited the clear function
and no separate documentation was necessary. Now, QMultiHash::clear
needs its own documentation. This patch adds it.

Task-number: 88533
Pick-to: 6.0
Change-Id: I93c59b66aa3d8ccf1888b6e24a4cc47004318e37
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/corelib/tools/qhash.cpp

@@ -2666,6 +2666,15 @@ size_t qHash(long double key, size_t seed) noexcept
     \sa remove()
 */
 
+/*!
+    \fn template <class Key, class T> void QMultiHash<Key, T>::clear()
+    \since 4.3
+
+    Removes all items from the hash and frees up all memory used by it.
+
+    \sa remove()
+*/
+
 /*! \fn template <class Key, class T> template <typename Predicate> qsizetype QMultiHash<Key, T>::removeIf(Predicate pred)
     \since 6.1