docs: es6ify 'Breaking Changes' and 'File' API pages (#15507)

* docs: apply arrow functions to app.makeSingleInstance example

* docs: apply arrow functions to session docs

* docs: change normal to arrow functions in File object docs

docs/api/breaking-changes.md

@@ -30,13 +30,13 @@ The following list includes the breaking API changes planned for Electron 4.0.
 
 ```js
 // Deprecated
-app.makeSingleInstance(function (argv, cwd) {
+app.makeSingleInstance((argv, cwd) => {
-
+  /* ... */
 })
 // Replace with
 app.requestSingleInstanceLock()
-app.on('second-instance', function (event, argv, cwd) {
+app.on('second-instance', (event, argv, cwd) => {
-
+  /* ... */
 })
 ```
 
@@ -212,11 +212,11 @@ screen.getPrimaryDisplay().workArea
 
 ```js
 // Deprecated
-ses.setCertificateVerifyProc(function (hostname, certificate, callback) {
+ses.setCertificateVerifyProc((hostname, certificate, callback) => {
   callback(true)
 })
 // Replace with
-ses.setCertificateVerifyProc(function (request, callback) {
+ses.setCertificateVerifyProc((request, callback) => {
   callback(0)
 })
 ```

docs/api/file-object.md

@@ -15,15 +15,15 @@ Example of getting a real path from a dragged-onto-the-app file:
 </div>
 
 <script>
-  document.addEventListener('drop', function (e) {
+  document.addEventListener('drop', (e) => {
     e.preventDefault();
     e.stopPropagation();
 
-    for (let f of e.dataTransfer.files) {
+    for (const f of e.dataTransfer.files) {
       console.log('File(s) you dragged here: ', f.path)
     }
   });
-  document.addEventListener('dragover', function (e) {
+  document.addEventListener('dragover', (e) => {
     e.preventDefault();
     e.stopPropagation();
   });