[3.14] Update struct.__doc__: _Bool available unconditionally (GH-143716) (#144070)

Update struct.__doc__: _Bool available unconditionally (GH-143716)

This amends commit a9296e7f3be4d6c22271b25c86467ff867c63bbb.
(cherry picked from commit 31c81ab0a2fb7009a76426617991a9b539ab0180)

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>