QMap::erase - extend docs to specify iterator limitations

Task-number: QTBUG-91736
Pick-to: 6.2 6.1
Change-Id: I4a226e0bbcde91f3149db9a0697981169ec25276
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/corelib/tools/qmap.qdoc

@@ -669,6 +669,8 @@
     from the map, and returns an iterator to the next item in the
     map.
 
+    \note The iterator \a pos \e must be valid and dereferencable.
+
     \sa remove()
 */
 
@@ -679,6 +681,8 @@
     [\a first, \a last) from the map.
     Returns an iterator to the item in the map following the last removed element.
 
+    \note The range \c {[first, last)} \e must be a valid range in \c {*this}.
+
     \sa remove()
 */