docs: document layers param, remove sources param documentation

[missinglink]

I noticed that the documentation mentions the sources param but not the layers param.

We'd like to continue supporting the layers param but recently started soft-deprecating the sources param, so we're removing mentions of sources= from all our documentation starting this month.

Rather than adding a new section for layers I've just rewritten the sources entry.
This commit covers both adding the new section and removing the old.