add a comment explaining motivation for @beikov's fix

Author: gavinking

hibernate-core/src/main/java/org/hibernate/internal/SessionImpl.java

@@ -2570,6 +2570,9 @@ public <T> T find(EntityGraph<T> entityGraph, Object primaryKey, FindOption... o
 	}
 
 	private void checkTransactionNeededForLock(LockMode lockMode) {
+		// OPTIMISTIC and OPTIMISTIC_FORCE_INCREMENT require a transaction
+		// because they involve a version check at the end of the transaction
+		// All flavors of PESSIMISTIC lock also clearly require a transaction
 		if ( lockMode.greaterThan( LockMode.READ ) ) {
 			checkTransactionNeededForUpdateOperation();
 		}