Support for sphinx-build v1.4.9 broken (v9.18 and v9.19)

Summary

Doc build fails with Sphinx 1.4.9. This is follow-up after #3295 (closed), going to even lower versions.

BIND version used

eaaf59d1

Steps to reproduce

• Install

alabaster (0.7.8)
Babel (2.3.4)
docutils (0.13.1)
imagesize (0.7.1)
Jinja2 (2.8)
MarkupSafe (0.23)
Pygments (2.2.0)
roman (2.0.0)
Sphinx (1.4.9)

• If on Python 3.10, tweak from collections import Mapping to from collections.abc
• make doc

What is the current bug behavior?

15:02:08 /build/bind9-9.18.2/doc/arm/manpages.rst:372: WARNING: Malformed option description '+[no]cdflag', should look like "opt", "-opt args", "--opt args", "/opt args" or "+opt args"

Possible fixes

Man pages for dig/mdig/delv use .. option:: +[no]bla and very old Sphinx does not like it.

After all it is a hack/shortcut which specifies hard to read strings things like +[no]subnet=addr[/prefix-length].

Proposed fix: Split "negative" and "positive" options. It will not only fix Sphinx build but also improve readability and ability to link to individual options from rest of the docs. Nobody is going to specify link targets like +[no]subnet ...