Is there documentation for the Rails column types?

I'm looking for more than the simple type listing that is found on this page:

:primary_key, :string, :text, :integer, :float, :decimal, :datetime, :timestamp, :time, :date, :binary, :boolean

But is there any documentation that actually defines these fields?

Specifically:

• What's the difference between :string and :text?
• Between :float and :decimal?
• What are the distinguishing features of :time, :timestamp, and :datetime?

Are the nuances of these types documented anywhere?

EDIT: Points of DB-platform implementations are irrelevant to the question I'm trying to ask. If, say, :datetime does not have a defined intended meaning in Rails documentation, then what do db-adapter-writers go by when choosing a corresponding column type?