Add `describe` operation to REPL server doc

[nvitucci]

I have noticed that the describe operation is not included in the REPL server doc. Any reasons for not adding it?

[daveyarwood]

I think I may have left out the describe, clone and eval ops from that doc, even though the server does support them, because I implemented them only for general adherence to the nREPL spec:

• https://nrepl.org/nrepl/1.0/building_clients.html
• https://nrepl.org/nrepl/1.0/ops.html

But, that said, I don't see any harm in documenting them. describe is useful in that it tells you the Alda version.

Thanks for the PR!