The NSS_STRICT_NOFORK environment variable is documented here: https://developer.mozilla.org/en/NSS_reference/NSS_environment_variables

Note that if a user really wants the strict pkcs11 behavior, the user can set the variable to "1" or some other non-empty value (other than "DISABLED").

If you think further documentation is required, I would be happy to update the Admin Guide, FAQ-o-matic, man pages, etc.