Resolve "asiodns.dox missing from tarball"

Closes #3433 (closed).

Here is the relevant part of the diff between the extracted tarballs.

$ /bin/diff -U0 -r before after
[...]
Only in after/src/lib/asiodns: asiodns.dox
[...]

A link to it was also missing from the main page of the developer's guide.

Before:

After:

• b31a5212 add asiodns.dox to tarball

• 7eee30a5 add libasiodns section