bpo-44693: Update __future__ entry in Doc/glossary.rst (GH-27349)

Replace sentence with confusing "pseudo-module" with two sentences
separating future statements and the __future__ module.

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Doc/glossary.rst

@@ -451,12 +451,13 @@ Glossary
       for best practices on working with annotations.
 
    __future__
-      A pseudo-module which programmers can use to enable new language features
-      which are not compatible with the current interpreter.
-
-      By importing the :mod:`__future__` module and evaluating its variables,
-      you can see when a new feature was first added to the language and when it
-      becomes the default::
+      A :ref:`future statement <future>`, ``from __future__ import <feature>``,
+      directs the compiler to compile the current module using syntax or
+      semantics that will become standard in a future release of Python.
+      The :mod:`__future__` module documents the possible values of
+      *feature*.  By importing this module and evaluating its variables,
+      you can see when a new feature was first added to the language and
+      when it will (or did) become the default::
 
          >>> import __future__
          >>> __future__.division

Misc/NEWS.d/next/Documentation/2021-07-25-23-04-15.bpo-44693.JuCbNq.rst

@@ -0,0 +1,2 @@
+Update the definition of __future__ in the glossary by replacing the confusing
+word "pseudo-module" with a more accurate description.