bpo-44693: Update __future__ entry in Doc/glossary.rst (GH-27349) (GH-27357)

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>
(cherry picked from commit 0363a4014d90df17a29042de008ef0b659f92505)

Co-authored-by: Steven Hsu <hsuhaochun@gmail.com>

Co-authored-by: Steven Hsu <hsuhaochun@gmail.com>