[3.9]bpo-45463: Clarify that global statements allows multiple names (GH-28851) ...

[3.9]bpo-45463: Clarify that global statements allows multiple names (GH-28851) (GH-28990)

The global statement allows specifying a list of identifiers
(https://docs.python.org/3/reference/simple_stmts.htmlGH-the-global-statement).

The "Execution model" chapter described the global statement as if it
only allowed one single name. Pluralize "name" in the appropriate places.
(cherry picked from commit 4ecd119b007cb766b8bede2dc78b70d29cd932dd)

Co-authored-by: Luca Chiodini <luca@chiodini.org>
Co-authored-by: Luca Chiodini <luca@chiodini.org>