Doc fixes

[postmodern]

Improve documentation of Async::DNS::Server.

• Removed old example for Server#initialize which referenced some kind of match method.
• Added a top-level example copied from the README.
• Added missing YARD tags.
• Fixed a YARD warning in lib/async/dns/transaction.rb.
• Tried my best to conform to your style (tab indentation, one-line @param tags, etc).

Types of Changes

• Documentation.

Contribution

• [x] I tested my changes locally.
• [x] I agree to the Developer's Certificate of Origin 1.1.

[ioquatix]

Thanks!