Update purple_contact_sort's documentation to say it returns < 0 and > 1

It turns out that g_utf8_collate will return values other than -1, 0, and 1 on
BSD which broke the unit tests which were expecting -1 but got -25.

Testing Done:
Ran the unit tests on FreeBSD and verified they passed.

Reviewed at https://reviews.imfreedom.org/r/2077/