Hygiene: enable syntax highlighting in JSDoc comments

Add the JSDoc Markdown plugin so that inline code snippets look their
very best. This plugin is currently used by the Popups repo and adding
it here also improves JSDoc config consistency.

Change-Id: Ib0094bed44268a1002d19ccb1a2495560331e09d

jsdoc.json

@@ -6,6 +6,9 @@
 		"readme": "README.md",
 		"recurse": true
 	},
+	"plugins": [
+		"plugins/markdown"
+	],
 	"source": {
 		"include": [ "resources" ]
 	},