added clarifying comment

Author: petrelharp

docs/metadata.md

@@ -580,6 +580,12 @@ Furthermore, these two sub-schemas must both define objects,
 and must not both define the same property:
 in other words, the  names of the properties in these must not overlap.
 
+When you use this codec, the decoded metadata in python is just a dictionary,
+as usual with either the JSON or struct codecs. There is no separation
+between the binary-encoded and JSON-encoded entries in metadata;
+this happens purely under the hood when encoding (and decoding) the metadata.
+See the example below.
+
 #### Binary representation
 
 The underlying structure of the JSON+struct codec is as follows.