[3.14] Add notes on nogil & reinitialization to the Opt-Out section in Module Isolation HOWTO (GH-134141) (GH-134492)

Add notes on nogil & reinitialization to the Opt-Out section in Module Isolation HOWTO (GH-134141)
(cherry picked from commit 1f0a294e8c2ff009c6b74ca5aa71da6269aec0dd)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Sam Gross <colesbury@gmail.com>