synchronize code and docs for `timestamps` and `add_timestamps`.

This makes the following changes:
  * warn if `:null` is not passed to `add_timestamps`
  * `timestamps` method docs link to `add_timestamps` docs
  * explain where additional options go
  * adjust examples to include `null: false` (to prevent deprecation warnings)

1 files changed, 5 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
index cc86c3776e..7cc5e666af 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
@@ -835,11 +835,14 @@ module ActiveRecord
         columns
       end
 
-      # Adds timestamps (+created_at+ and +updated_at+) columns to the named table.
+      include TimestampDefaultDeprecation
+      # Adds timestamps (+created_at+ and +updated_at+) columns to +table_name+.
+      # Additional options (like <tt>null: false</tt>) are forwarded to #add_column.
       #
-      #   add_timestamps(:suppliers)
+      #   add_timestamps(:suppliers, null: false)
       #
       def add_timestamps(table_name, options = {})
+        emit_warning_if_null_unspecified(options)
         add_column table_name, :created_at, :datetime, options
         add_column table_name, :updated_at, :datetime, options
       end