doc: improve dns introduction

The introductory paragraphs for the dns module do not explain what the module is for. Add a short (two brief sentences) explanation.

It also emphasizes that functions in the dns module are divided into two categories, but that there is only one function in one of the categories, making the whole categories thing a bit odd to emphasize. Keep the material about the distinctions without discussing categories.

There are other incidental revisions (changing an example IP to example.org and so on).

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines