nroff output isn't aware of ordered list numeration type

Hiya,

I've run into an issue where we are seeing output from AsciiDoc->nroff
producing the wrong type of numerated list. Specifically, when the
AsciiDoc uses a lettered list like so:

```
A submodule is considered active,

  a. if `submodule.<name>.active` is set to `true`
+
or

  b. if the submodule's path matches the pathspec in `submodule.active`
+
or

  c. if `submodule.<name>.url` is set.

and these are evaluated in this order.
```
The formatted nroff, opened with `man`, looks like so:

```

       A submodule is considered active,

        1. if submodule.<name>.active is set to true

           or

        2. if the submodule’s path matches the pathspec in submodule.active

           or

        3. if submodule.<name>.url is set.

       and these are evaluated in this order.
```
(Bold excluded from above due to plaintext email mode)

However, HTML generated from the same source AsciiDoc gives a lettered
list as expected.

You can see examples of the disparity in the document this bug is
based on, by examining https://gitirc.eu/gitsubmodules.html or by
running `man gitsubmodules` if your copy of Git is version 2.19.2 or
newer.

My version of asciidoc is 8.7.10; I'm not sure where I would find
which schema version we're using elsewhere but I'm happy to look it
up.

Thank you!
Emily Shaffer