Expand the docs of ScenarioInfo

[glatterf42]

Should we leave a note somewhere that we strongly encourage users to still use the syntax you were trying to enforce?

We could in fact substantially expand the docs of ScenarioInfo with narrative explanation and examples of the many ways the class can be/is used in message-ix-models and message_data. For instance, see how doc/api/util.rst has extra content on how to use the Context class.

Whether to do that in this PR or another, I leave to you.

Originally posted by @khaeru in #146 (review)