Add documentation for IndexDefinition.

activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb

@@ -6,6 +6,9 @@ require 'bigdecimal/util'
 
 module ActiveRecord
   module ConnectionAdapters #:nodoc:
+    # Abstract representation of an index definition on a table. Instances of
+    # this type are typically created and returned by methods in database
+    # adapters. e.g. ActiveRecord::ConnectionAdapters::AbstractMysqlAdapter#indexes
     class IndexDefinition < Struct.new(:table, :name, :unique, :columns, :lengths, :orders, :where) #:nodoc:
     end