Mention how to test third party stubs (#13505)

Fixes #12241

Author: hauntsaninja

docs/source/command_line.rst

@@ -830,7 +830,8 @@ in developing or debugging mypy internals.
     submitting them upstream, but also allows you to use a forked version of
     typeshed.
 
-    Note that this doesn't affect third-party library stubs.
+    Note that this doesn't affect third-party library stubs. To test third-party stubs,
+    for example try ``MYPYPATH=stubs/six mypy ...``.
 
 .. _warn-incomplete-stub:
 

docs/source/config_file.rst

@@ -858,9 +858,16 @@ These options may only be set in the global section (``[mypy]``).
 
     :type: string
 
-    Specifies an alternative directory to look for stubs instead of the
-    default ``typeshed`` directory. User home directory and environment
-    variables will be expanded.
+    This specifies the directory where mypy looks for standard library typeshed
+    stubs, instead of the typeshed that ships with mypy.  This is
+    primarily intended to make it easier to test typeshed changes before
+    submitting them upstream, but also allows you to use a forked version of
+    typeshed.
+
+    User home directory and environment variables will be expanded.
+
+    Note that this doesn't affect third-party library stubs. To test third-party stubs,
+    for example try ``MYPYPATH=stubs/six mypy ...``.
 
 .. confval:: warn_incomplete_stub