gh-89554: Document socket.SocketType as a class (#150683)

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::", correct the misleading description
(SocketType is the base class of the socket type, not "type(socket(...))"
which is socket.socket; addresses gh-88427), move it into the Socket
Objects section, and document the socket object methods and attributes
nested under the socket class, dropping the redundant "socket." prefix.

Author: gaborbernat