[3.15] gh-89554: Document socket.SocketType as a class (GH-150683) (#151244)

gh-89554: Document socket.SocketType as a class (GH-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.
(cherry picked from commit a621e8a)

Co-authored-by: Bernát Gábor <gaborjbernat@gmail.com>

Author: miss-islington