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/turtle.rst b/Doc/library/turtle.rst
index fed8504..d348753 100644
--- a/Doc/library/turtle.rst
+++ b/Doc/library/turtle.rst
@@ -1069,6 +1069,7 @@
 ~~~~~~~~~~~~~~~~~~~~
 
 .. function:: reset()
+   :noindex:
 
    Delete the turtle's drawings from the screen, re-center the turtle and set
    variables to the default values.
@@ -1090,6 +1091,7 @@
 
 
 .. function:: clear()
+   :noindex:
 
    Delete the turtle's drawings from the screen.  Do not move turtle.  State and
    position of the turtle as well as drawings of other turtles are not affected.
@@ -1362,6 +1364,7 @@
 ------------
 
 .. function:: onclick(fun, btn=1, add=None)
+   :noindex:
 
    :param fun: a function with two arguments which will be called with the
                coordinates of the clicked point on the canvas