[3.13] gh-126720: Docs: Miscellaneous corrections to simple statements in the language reference (GH-126720) (#126885)

Docs: Miscellaneous corrections to simple statements in the language reference (GH-126720)

* Replace: The :keyword:`global` -> The :keyword:`global` statement
Add :keyword: when it's needed

* Replace repeated links with duoble backticks

(cherry picked from commit 94a7a4e22fb8f567090514785c69e65298acca42)

Co-authored-by: Beomsoo Kim <beoms424@gmail.com>