devel/nts.adoc edits

[James Browning]

I have an HTML/SVG render of a modified devel/nts.adoc at:

http://www.jamesb192.com:8001/james/nts.html

The source in in MR !894 at:

https://gitlab.com/NTPsec/ntpsec/merge_requests/894

I would appreciate it if people would look at it and provide constructive
criticism about the content. The edits are sorted roughly into things I
think are acceptable as is, acceptable with some revision and thing I
think I need to seriously edit or drop. roughly in that order