bpo-40204: Add :noindex: in the documentation (GH-21859)

Add :noindex: to duplicated documentation to fix "duplicate object
description" errors.

For example, fix this Sphinx 3 issue:

Doc/library/configparser.rst:1146: WARNING: duplicate object
description of configparser.ConfigParser.optionxform, other instance
in library/configparser, use :noindex: for one of them
diff --git a/Doc/library/socket.rst b/Doc/library/socket.rst
index d798c1a..5bcac20 100755
--- a/Doc/library/socket.rst
+++ b/Doc/library/socket.rst
@@ -1695,7 +1695,9 @@
 
 .. method:: socket.setsockopt(level, optname, value: int)
 .. method:: socket.setsockopt(level, optname, value: buffer)
+   :noindex:
 .. method:: socket.setsockopt(level, optname, None, optlen: int)
+   :noindex:
 
    .. index:: module: struct