umb.4: Improve synposis, add netmap

Rewrite synopsis in a more consistent style, and add "device netmap",
In FreeBSD 15, netmap is included in GENERIC/{amd64,arm64,powerpc64},
but it still needs to be used in FreeBSD 14 or custom kernels which
don't include GENERIC.

MFC after: umb does not mfc
Reviewed by: gbe
Differential Revision: https://reviews.freebsd.org/D51732