QMultiMap::erase - extend docs to specify iterator limitations

Task-number: QTBUG-91736
Change-Id: I7cd58e010af5dd59404e37c55f6ebd91c4631b3f
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
(cherry picked from commit d1d9caf12d103957dccc867c349c4514a28cfe6c)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/tools/qmultimap.qdoc

@@ -699,6 +699,8 @@
     from the multi map, and returns an iterator to the next item in the
     map.
 
+    \note The iterator \a pos must be valid and dereferenceable.
+
     \sa remove()
 */
 
@@ -710,6 +712,8 @@
     Returns an iterator to the item in the multi map following the last
     removed element.
 
+    \note The range \c {[first, last)} \e must be a valid range in \c {*this}.
+
     \sa remove()
 */