Use the externally_connectable schema from manifest_types.json to generate its

chrome/common/extensions/docs/templates/json/manifest.json

 {
   "app": {
     "documentation": "manifest/app.html",
     "example": {},
     "level": "required"
   },
   "app.background": {
     "example": {"scripts": ["background.js"]},
     "level": "required"
   },
@@ skipping 26 matching lines @@
     "documentation": "manifest/default_locale.html",
     "example": "en",
     "level": "recommended"
   },
   "description": {
     "documentation": "manifest/description.html",
     "example": "A plain text description",
     "level": "recommended"
   },
   "externally_connectable": {
-    "documentation": "messaging.html#external-webpage",
+    "documentation": "manifest/externally_connectable.html",
     "example": {
       "matches": ["*://*.example.com/*"]
     }
   },
   "file_browser_handlers": {
     "documentation": "fileBrowserHandler.html",
     "example": []
   },
   "file_handlers": {
     "documentation": "manifest/file_handlers.html",
@@ skipping 85 matching lines @@
   "version": {
     "documentation": "manifest/version.html",
     "example": "versionString",
     "level": "required"
   },
   "web_accessible_resources": {
     "documentation": "manifest/web_accessible_resources.html",
     "example": []
   }
 }