docs: add latest bug to known problems for 2.1.5

documentation/KnownProblems.txt

 Known Problems
 ==============
 
+.. _Known_Problems_2.1.5:
+
+Known Problems in Release 2.1.5
+-------------------------------
+
+* Using pvPutComplete or pvGetComplete inside a when-clause may
+  result in a deadlock: the state set forever waits for the
+  completion even though the callback arrived.
+
+  Note: this Problem is present at least since 2.0.12.
+
+
 .. _Known_Problems_2.1.4:
 
 Known Problems in Release 2.1.4

documentation/Notes.txt

@@ -6,6 +6,8 @@ Release Notes
 Release 2.1.5
 -------------
 
+:ref:`Known_Problems_2.1.5`
+
 * fixed two bugs in pvAssign:
 
   - after an initially unassigned variable gets assigned using