Document SocketType as a class without restructuring the socket methods

Drop the Socket Objects restructure and the second `.. class:: socket` that
required `:noindex:`. Keep only the change of `SocketType` from `.. data::` to
`.. class::`, in its original place after the `socket` class, with a corrected
description: SocketType is the base class of socket re-exported from _socket,
not `type(socket(...))`.

Author: gaborbernat