Document how FlushStateMode::NONE is handled

src/validation.cpp

@@ -2064,6 +2064,9 @@ bool CChainState::ConnectBlock(const CBlock& block, CValidationState& state, CBl
  * The caches and indexes are flushed depending on the mode we're called with
  * if they're too large, if it's been a while since the last write,
  * or always and in all cases if we're in prune mode and are deleting files.
+ *
+ * If FlushStateMode::NONE is used, then FlushStateToDisk(...) won't do anything
+ * besides checking if we need to prune.
  */
 bool static FlushStateToDisk(const CChainParams& chainparams, CValidationState &state, FlushStateMode mode, int nManualPruneHeight) {
     int64_t nMempoolUsage = mempool.DynamicMemoryUsage();