DOCSP-14244 add reference to db.collection.validate()

Author: ajhuh-mdb

source/core/schema-validation.txt

@@ -137,6 +137,11 @@ Validation occurs during updates and inserts. When you add validation to
 a collection, existing documents do not undergo validation checks until
 modification.
 
+To perform validation checks on existing documents, use the
+:dbcommand:`validate` command or the :method:`db.collection.validate()` 
+shell helper.
+
+
 Existing Documents
 ~~~~~~~~~~~~~~~~~~
 
@@ -306,4 +311,5 @@ Additional Information
 ----------------------
 
 .. seealso:: :dbcommand:`collMod`, :method:`db.createCollection()`,
-   :method:`db.getCollectionInfos()`.
+   :method:`db.getCollectionInfos()`, 
+   :method:`db.collection.validate()`.