add more explanation to CollectionAssociation docs

activerecord/lib/active_record/associations/collection_association.rb

@@ -11,6 +11,10 @@ module Associations
     #     HasManyAssociation => has_many
     #       HasManyThroughAssociation + ThroughAssociation => has_many :through
     #
+    # CollectionAssociation class provides common methods to the collections
+    # defined by +has_and_belongs_to_many+, +has_many+ and +has_many+ with
+    # +:through+ association option.
+    #
     # You need to be careful with assumptions regarding the target: The proxy
     # does not fetch records from the database until it needs them, but new
     # ones created with +build+ are added to the target. So, the target may be