Fix parameter names in assertIn() docs (GH-18829)

The names "member" and "container" for the arguments are also used in the module and shown with the help() function, and are immediately understandable in this context, contrary to "first" and "second".
(cherry picked from commit a388bbd3f129364c39843f63e92f08bc53c71905)

Co-authored-by: Christoph Zwerschke <cito@online.de>

diff --git a/Doc/library/unittest.rst b/Doc/library/unittest.rst
index a559d0988bee2ef2c57e99b5223857eba144d49f..d9f080a862ff25344755eacf636b2950024b2a3d 100644 (file)
--- a/Doc/library/unittest.rst
+++ b/Doc/library/unittest.rst
@@ -910,10 +910,10 @@ Test cases
       .. versionadded:: 3.1
 
 
-   .. method:: assertIn(first, second, msg=None)
-               assertNotIn(first, second, msg=None)
+   .. method:: assertIn(member, container, msg=None)
+               assertNotIn(member, container, msg=None)
 
-      Test that *first* is (or is not) in *second*.
+      Test that *member* is (or is not) in *container*.
 
       .. versionadded:: 3.1