Update doc for sk_TYPE_find() and sk_TYPE_find_ex()

to better describe the vagaries in their behaviour.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2958)
(cherry picked from commit 89b06ca7)