doc: reword external

Signed-off-by: Simon de Vlieger <[email protected]>

doc/03-omnifest/02-external.md

@@ -31,7 +31,8 @@ The external gets called with the following JSON:
 }
 ```
 
-And is expected to return the following JSON:
+And is expected to return a JSON dict with a single top-level `tree` object that
+can contain arbitrary values. Additional top-level keys are not allowed. Example:
 
 ```json
 {
@@ -43,28 +44,28 @@ And is expected to return the following JSON:
 }
 ```
 
-Which will lead to the following YAML structure:
+Which will replace the previous `otk.external.name` subtree with the output of a
+new subtree from the external comand. Example
 
 ```yaml
-tree:
-  what:
-    you: "want"
+what:
+  you: "want"
 ```
 
 If the external returns `{}`, an empty object, `otk` will assume that there is
 no tree to replace and remove the node instead. This will turn the following
 YAML:
 
 ```yaml
-dummy:
+example:
   otk.external.name:
     options:
 ```
 
 Into this YAML structure:
 
 ```yaml
-dummy:
+example:
 ```
 
 ## Paths