Merge pull request #21623 from schneems/schneems/schema-migration-docs

Doc SchemaMigration class.

activerecord/lib/active_record/schema_migration.rb

@@ -2,7 +2,11 @@
 require 'active_record/scoping/named'
 
 module ActiveRecord
-  class SchemaMigration < ActiveRecord::Base
+  # This class is used to create a table that keeps track of which migrations
+  # have been applied to a given database. When a migration is run, its schema
+  # number is inserted in to the `SchemaMigration.table_name` so it doesn't need
+  # to be executed the next time.
+  class SchemaMigration < ActiveRecord::Base # :nodoc:
     class << self
       def primary_key
         nil