(GH-53) Add consistency section

Author: shorodilov

src/rdbms/acid.txt

@@ -61,9 +61,13 @@ fails.
 Consistency
 ===========
 
-.. todo: help ensure the data to be in correct state,
-         data integrity constraints must be followed.
-         consistency = correctness
+You can think "consistency" as "correctness". It ensures that a transaction
+can only bring the database from one consistent state to another, preserving
+database invariants: any data written to the database must be valid according
+to all defined rules, including constraints, cascades, triggers, and any
+combination of thereof. This prevents database corruption by an illegal
+transaction. As example, referential integrity guarantees the primary key --
+foreign key relationship.
 
 Isolation
 =========