Add comment to describe callers to ActivateBestChain

src/validation.cpp

@@ -2388,6 +2388,11 @@ static void NotifyHeaderTip() {
  * that is already loaded (to avoid loading it again from disk).
  */
 bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams, std::shared_ptr<const CBlock> pblock) {
+    // Note that while we're often called here from ProcessNewBlock, this is
+    // far from a guarantee. Things in the P2P/RPC will often end up calling
+    // us in the middle of ProcessNewBlock - do not assume pblock is set
+    // sanely for performance or correctness!
+
     CBlockIndex *pindexMostWork = NULL;
     CBlockIndex *pindexNewTip = NULL;
     do {