Add a JSON config file example for the CLI docs

docs/cli.md

@@ -160,9 +160,23 @@ myext:
     option2: True
 ```
 
+Similarly, a JSON configuration file might look like this:
+
+```json
+{
+  "myext":
+  {
+    "option1": "value1",
+    "option2": "value2"
+  }
+}
+```
+
 Note that while the `--extension_configs` option does specify the
 `myext` extension, you still need to load the extension with the `-x` option,
-or the configuration for that extension will be ignored.
+or the configuration for that extension will be ignored. Further, if an
+extension requires a value that cannot be parsed in JSON (for example a
+reference to a function), one has to use a YAML configuration file.
 
 The `--extension_configs` option will only support YAML configuration files if
 [PyYAML] is installed on your system. JSON should work with no additional