QSet::erase - extend docs

Explicitly specify that calling this method for an empty set or with
an invalid iterator results in undefined behavior.

On a debug build an assert is triggered in such case, but on a release
build it will access the incorect index of an array.

Task-number: QTBUG-91736
Pick-to: 6.2 6.1
Change-Id: Ibc3e91512a0ad9d9779a41083fedb8a91780380b
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/tools/qset.qdoc

@@ -275,6 +275,11 @@
     internal data structure. This means that it can safely be called
     while iterating, and won't affect the order of items in the set.
 
+    \note The iterator \a pos \e must be valid and dereferenceable. Calling this
+    method on any other iterator, including its own \l end(), results in
+    undefined behavior. In particular, even the \l begin() iterator of an empty
+    set cannot be dereferenced.
+
     \sa remove(), find()
 */