gh-89554: Document socket.SocketType as a class and fix its description

socket.SocketType is a class (re-exported from _socket as an alias of
_socket.socket, the base class of socket.socket), but was documented with
the ".. data::" directive, so ":class:" cross-references to it cannot
resolve against a py:class target.

Switch the entry to ".. class::", and correct the description: SocketType
is the base class of the socket type, not "type(socket(...))" (which is
socket.socket). This addresses the misleading wording reported in
gh-88427.

Author: gaborbernat

Doc/library/socket.rst

@@ -1025,10 +1025,12 @@ The following functions all create :ref:`socket objects <socket-objects>`.
    .. versionadded:: 3.3
 
 
-.. data:: SocketType
+.. class:: SocketType
 
-   This is a Python type object that represents the socket object type. It is the
-   same as ``type(socket(...))``.
+   The base class of the :class:`~socket.socket` type, re-exported from
+   :mod:`!_socket`.  An instance check such as
+   ``isinstance(socket(...), SocketType)`` is true, but ``SocketType`` is not
+   the same as ``type(socket(...))``, which is :class:`~socket.socket` itself.
 
 
 Other functions