Document metadata schema used for populations

[hyanwong]

I think by default when running sim_ancestry we set ts.tables.populations.metadata_schema to something that requires a name and description. If I do a search in the docs for "schema", I can't find anywhere that this is actually documented though?

[jeromekelleher]

It's not documented no - should be. I decided that there was some extra infrastructure needed in tskit to shield users from the details of JSON schema and didn't go any further than defining the schema outline, e.g. things like having a metadata schema API rather than making them write out JSON by hand, and perhaps https://github.com/tskit-dev/tskit/issues/1092