Document Fleece's nesting limit when users create a Query

A client saw the issue of creating a query with more than 50 levels of nesting when encoded leading to JSON parser error (CBSE-6962).  This nesting limit of the JSON parser is set in JSONConverter.cc: https://github.com/couchbase/fleece/blob/b11812cbe7d31daee467125bd64a9b3fb0a5337f/Fleece/Core/JSONConverter.cc#L35,but it is not documented anywhere, which might cause the same issue for users in the future. Therefore, there needs to be a note on this limit for users to keep in mind when creating a query.