Update ManualMocks docs.

Summary: Closes jestjs#860

Differential Revision: D3132497

fb-gh-sync-id: 3a14b16aad86f3fc5f7a22e4dfc1a9ee14d0e944
fbshipit-source-id: 3a14b16aad86f3fc5f7a22e4dfc1a9ee14d0e944

Author: cpojer

docs/ManualMocks.md

@@ -64,6 +64,8 @@ that we can build on for our tests:
 
 const path = require('path');
 
+const fs = jest.genMockFromModule('fs');
+
 // This is a custom function that our tests can use during setup to specify
 // what the files on the "mock" filesystem should look like when any of the
 // `fs` APIs are used.
@@ -86,8 +88,10 @@ function readdirSync(directoryPath) {
   return mockFiles[directoryPath] || [];
 }
 
-exports.__setMockFiles = __setMockFiles;
-exports.readdirSync = readdirSync;
+fs.__setMockFiles = __setMockFiles;
+fs.readdirSync = readdirSync;
+
+module.exports = fs;
 ```
 
 Now we write our test:
@@ -130,7 +134,7 @@ The example mock shown here uses [`jest.genMockFromModule`](/jest/docs/api.html#
 to generate an automatic mock, and overrides its default behavior. This is the
 recommended approach, but is completely optional. If you do not want to use the
 automatic mock at all, you can simply export your own functions from the mock
-file. Of course, one downside to fully manual mocks is that they're manual –
+file. One downside to fully manual mocks is that they're manual –
 meaning you have to manually update them any time the module they are mocking
 changes. Because of this, it's best to use or extend the automatic mock when it
 works for your needs.

examples/manual_mocks/__mocks__/fs.js

@@ -2,6 +2,8 @@
 
 const path = require('path');
 
+const fs = jest.genMockFromModule('fs');
+
 // This is a custom function that our tests can use during setup to specify
 // what the files on the "mock" filesystem should look like when any of the
 // `fs` APIs are used.
@@ -24,6 +26,7 @@ function readdirSync(directoryPath) {
   return mockFiles[directoryPath] || [];
 }
 
-exports.__setMockFiles = __setMockFiles;
-exports.readdirSync = readdirSync;
+fs.__setMockFiles = __setMockFiles;
+fs.readdirSync = readdirSync;
 
+module.exports = fs;