mbuf.9: Document mtodo

mtodo() accepts an mbuf and offset and returns a void * pointer to the
requested offset into the mbuf's associated data. Similar to mtod(),
no bounds checking is performed.

Reviewed by: imp, markj
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D43215

(cherry picked from commit 854e1fd692185749fea092b1431efd4a8e09c2eb)