geom_zero.4: Document

PR: 250593
Reviewed by: bcr, ziaee
Thanks to: imp, markj
MFC after: 1 week
Fixes: 3843eba85d98 Add unmapped BIO support to GEOM ZERO
Fixes: 24e1fdcd1a69 Allow to specify the byte which will be used for filling read buffer
Fixes: 565bc101112c Add a very simple and small GEOM class - ZERO