bpo-37765: Add keywords to IDLE tab completions (GH-15138)

Keywords are present in the main module tab completion lists generated by rlcompleter, which is used by REPLs on *nix. Add all keywords to IDLE's main module name list except those already added from builtins (True, False, and None) . This list may also be used by Show Completions on the Edit menu, and its hot key.

Rewrite Completions doc.

Co-authored-by: Cheryl Sabella <cheryl.sabella@gmail.com>