Update documentation for "file_system_provider" manifest section.

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

 {
   "app": {
     "documentation": "manifest/app",
     "example": {},
     "level": "required"
   },
   "app.background": {
     "example": {"scripts": ["background.js"]},
     "level": "required"
   },
@@ skipping 70 matching lines @@
     }
   },
   "file_browser_handlers": {
     "documentation": "fileBrowserHandler",
     "example": []
   },
   "file_handlers": {
     "documentation": "manifest/file_handlers",
     "example": {}
   },
+  "file_system_provider": {
+    "documentation": "manifest/file_system_provider",
+    "example": {
+      "configurable": true,
+      "multiple_mounts": true,
+      "source": "network"
+    }
+  },
   "homepage_url": {
     "documentation": "manifest/homepage_url",
     "example": "http://path/to/homepage"
   },
   "icons": {
     "documentation": "manifest/icons",
     "example": {},
     "level": "recommended"
   },
   "import": {
@@ skipping 124 matching lines @@
   },
   "web_accessible_resources": {
     "documentation": "manifest/web_accessible_resources",
     "example": []
   },
   "webview": {
     "documentation": "tags/webview#local_resources",
     "example": {}
   }
 }