:envvar: is currently used to tag environment variables, however,
:envvar: role expects a matching .. envvar:: directive where the
environment variable is defined. This patch fixes some cases by adding
envvar directives and by converting certain cases to literals (it
doesn't make sense to document all env vars in a list, as some are
self-documented by contextual information).
Ref. https://www.sphinx-doc.org/en/master/usage/restructuredtext/
roles.html#role-envvar
Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>
