Document validationinterace callback blocking deadlock potential.

src/validation.cpp

@@ -2704,6 +2704,9 @@ bool CChainState::ActivateBestChain(CValidationState &state, const CChainParams&
             // Block until the validation queue drains. This should largely
             // never happen in normal operation, however may happen during
             // reindex, causing memory blowup if we run too far ahead.
+            // Note that if a validationinterface callback ends up calling
+            // ActivateBestChain this may lead to a deadlock! We should
+            // probably have a DEBUG_LOCKORDER test for this in the future.
             SyncWithValidationInterfaceQueue();
         }