Verify that all APIs are documented in unit test

Summary: When exposing new top-level APIs from `flipper-plugin`, they should be documented. Added a unit test to enforce this and added documentation for all missing APIs.

Reviewed By: passy

Differential Revision: D25421401

fbshipit-source-id: f5cafc1881de846c8a5dd86e5d094ebd27a66f2a

desktop/flipper-plugin/src/__tests__/api.node.tsx

@@ -7,6 +7,8 @@
  * @format
  */
 
+import {readFile} from 'fs';
+import {promisify} from 'util';
 import * as FlipperPluginModule from '../index';
 
 test('Correct top level API exposed', () => {
@@ -67,3 +69,21 @@ test('Correct top level API exposed', () => {
     ]
   `);
 });
+
+test('All APIs documented', async () => {
+  const docs = await promisify(readFile)(
+    __dirname + '/../../../../docs/extending/flipper-plugin.mdx',
+    'utf8',
+  );
+  Object.keys(FlipperPluginModule)
+    .filter(
+      (key) =>
+        !key.startsWith('_') && (FlipperPluginModule as any)[key] !== undefined,
+    )
+    .forEach((key) => {
+      // There should be a header with this identifier
+      if (!new RegExp(`# ${key}\\b`).test(docs)) {
+        fail(`Not documented: '${key}'`);
+      }
+    });
+});