Document TOR_PT_EXIT_ON_STDIN_CLOSE in the pt-spec.

This is the ticket for the documentation side of the TOR_PT_EXIT_ON_STDIN_CLOSE and associated behavior that was implemented as part of #15435 (moved).

pt-spec.txt needs to document that if the env var is set to 1, then PTs should assume that tor will keep stdin open, and to treat stdin being closed as the same as a SIGTERM (graceful shutdown immediately).