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
Change-Id: I93c59b66aa3d8ccf1888b6e24a4cc47004318e37
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 3c0614b7794d917045a496144df506eeeb2a57c6)
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/tools/qhash.cpp

@@ -2646,6 +2646,15 @@ 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> T QMultiHash<Key, T>::take(const Key &key)
 
     Removes the item with the \a key from the hash and returns