signal: Remove gsignal references from manuals

Reported by: Mina Galić, mhorne
Reviewed by: Mina Galić, mhorne
Differential Revision: https://reviews.freebsd.org/D41020
MFC after: 1 week

(cherry picked from commit c3f58005d92e5ac189e7dcb2d6d868ed5a0f27fa)