Add an overview section to bus_dma.9.

Describe the role of tags and mapping objects as abstractions.
Describe static vs dynamic transaction types and give a brief overview
of the set of functions and object life cycles used for static vs
dynamic.

While here, fix a few other typos and expand a bit on parent tags.

Reviewed by: cem, imp
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D16652