docs: document packages, exported API and non-obvious logic

internal/deezer/resource.go

@@ -1,5 +1,14 @@
 package deezer
 
+// Resource is a downloadable Deezer page: an Album, Playlist, Artist or
+// Single. Implementations differ only in how they decode the gw-light
+// response and where they place their output, so the accessors are
+// intentionally thin and are not documented individually.
+//
+// The unexported decode method seals the interface. Only the four types in
+// this package can satisfy it, which lets Kind.newResource stay an exhaustive
+// switch and guarantees FetchResource never receives an implementation whose
+// wire format it does not know.
 type Resource interface {
 	Title() string
 	Tracks() []*Track