Update Soy docs to state when a default deltemplate is required.
@@ -75,10 +75,14 @@ with the highest priority. Basically, this means:
2. Otherwise, use the default implementation, if there is one.
3. Otherwise, if the `delcall` has the attribute `allowemptydefault="true"`,
then the call renders to the empty string.
-4. Otherwise, an error occurs.
Due to the third case, it is legal to render a delegate template that has no
default implementation, as long as the `delcall` has `allowemptydefault="true"`.
+Without `allowemptydefault="true"`, a default implementation must exist and be
+in the dependencies of the file containing the `delcall`.
+
+Important: `allowemptydefault="true"` is deprecated and support will be removed
+soon. You should always provide a default implementation, even if it's empty.
What counts as an "active" implementation depends on the backend in use. In
JavaScript, an active implementation is simply an implementation that is defined
