Document data availability queries

[khaeru]

#161 added support for these queries. One example is given in #161 (comment), but these should also be documented, because of their (possibly) non-intuitive semantics: they use a data-like query and key, but return a StructureMessage with the AvailableConstraint and other artefacts.

Since there is no known public SDMX REST data sources that support such queries:

• Identify (e.g. by manually checking those in sources.json) or monitor for such a source.
• Add an example to the documentation of how to make such a query.