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

Author: ajhuh-mdb

source/core/schema-validation.txt

@@ -148,6 +148,11 @@ 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
 ~~~~~~~~~~~~~~~~~~
 
@@ -423,3 +428,4 @@ Additional Information
    - :dbcommand:`collMod`
    - :method:`db.createCollection()`
    - :method:`db.getCollectionInfos()`
+   - :method:`db.collection.validate()`