Add detailed helptext for the request command

[za419]

This will likely be the second highest complexity helptext. Add an entry into default-config.json under helpTopics. The key should probably be 'request', and the description should describe how one can use the command and how it will behave from a user perspective.

That means this needs to document both the 'traditional' request from an index out of search or library, and the newfangled one-step-request flow ('newfangled' here meaning "only two thirds of the bot's life old", as documented by #11 and resolved in #19 and #20 ).

The latter should definitely reference the helptext for search (#71 ) rather than redocument search for one-step-request.