g_bio(9): add link to bio(9)

This page documents 'struct bio', so this can be helpful with finding or
referencing it.

Reviewed by: imp, markj
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41252

(cherry picked from commit d441ec1c218991421d08ac4661878ed5c182cd64)