doc: update and modernize examples in fs.ms

* unify quotes in fs.md
* avoid quote escaping in fs.md
* simplify logics in fs.md
* concatenation -> template literal in fs.md
* add missing callback in fs.md
* fix typo in fs.md
* update output example in fs.md

PR-URL: #12035
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/fs.md

@@ -218,7 +218,7 @@ For a regular file [`util.inspect(stats)`][] would return a string very
 similar to this:
 
 ```js
-{
+Stats {
   dev: 2114,
   ino: 48064969,
   mode: 33188,
@@ -232,8 +232,7 @@ similar to this:
   atime: Mon, 10 Oct 2011 23:24:11 GMT,
   mtime: Mon, 10 Oct 2011 23:24:11 GMT,
   ctime: Mon, 10 Oct 2011 23:24:11 GMT,
-  birthtime: Mon, 10 Oct 2011 23:24:11 GMT
-}
+  birthtime: Mon, 10 Oct 2011 23:24:11 GMT }
 ```
 
 Please note that `atime`, `mtime`, `birthtime`, and `ctime` are
@@ -377,12 +376,12 @@ fs.access('myfile', (err) => {
 ```js
 fs.open('myfile', 'wx', (err, fd) => {
   if (err) {
-    if (err.code === "EEXIST") {
+    if (err.code === 'EEXIST') {
       console.error('myfile already exists');
       return;
-    } else {
-      throw err;
     }
+
+    throw err;
   }
 
   writeMyData(fd);
@@ -394,12 +393,12 @@ fs.open('myfile', 'wx', (err, fd) => {
 ```js
 fs.access('myfile', (err) => {
   if (err) {
-    if (err.code === "ENOENT") {
+    if (err.code === 'ENOENT') {
       console.error('myfile does not exist');
       return;
-    } else {
-      throw err;
     }
+
+    throw err;
   }
 
   fs.open('myfile', 'r', (err, fd) => {
@@ -414,12 +413,12 @@ fs.access('myfile', (err) => {
 ```js
 fs.open('myfile', 'r', (err, fd) => {
   if (err) {
-    if (err.code === "ENOENT") {
+    if (err.code === 'ENOENT') {
       console.error('myfile does not exist');
       return;
-    } else {
-      throw err;
     }
+
+    throw err;
   }
 
   readMyData(fd);
@@ -729,13 +728,14 @@ fs.exists('myfile', (exists) => {
 ```js
 fs.open('myfile', 'wx', (err, fd) => {
   if (err) {
-    if (err.code === "EEXIST") {
+    if (err.code === 'EEXIST') {
       console.error('myfile already exists');
       return;
-    } else {
-      throw err;
     }
+
+    throw err;
   }
+
   writeMyData(fd);
 });
 ```
@@ -759,15 +759,15 @@ fs.exists('myfile', (exists) => {
 ```js
 fs.open('myfile', 'r', (err, fd) => {
   if (err) {
-    if (err.code === "ENOENT") {
+    if (err.code === 'ENOENT') {
       console.error('myfile does not exist');
       return;
-    } else {
-      throw err;
     }
-  } else {
-    readMyData(fd);
+
+    throw err;
   }
+
+  readMyData(fd);
 });
 ```
 
@@ -945,7 +945,7 @@ const fd = fs.openSync('temp.txt', 'r+');
 
 // truncate the file to 10 bytes, whereas the actual size is 7 bytes
 fs.ftruncate(fd, 10, (err) => {
-  assert.ifError(!err);
+  assert.ifError(err);
   console.log(fs.readFileSync('temp.txt'));
 });
 // Prints: <Buffer 4e 6f 64 65 2e 6a 73 00 00 00>
@@ -1154,8 +1154,8 @@ fs.mkdtemp(tmpDir, (err, folder) => {
 });
 
 // This method is *CORRECT*:
-const path = require('path');
-fs.mkdtemp(tmpDir + path.sep, (err, folder) => {
+const { sep } = require('path');
+fs.mkdtemp(`${tmpDir}${sep}`, (err, folder) => {
   if (err) throw err;
   console.log(folder);
   // Will print something similar to `/tmp/abc123`.
@@ -1564,7 +1564,7 @@ argument will automatically be normalized to absolute path.
 Here is an example below:
 
 ```js
-fs.symlink('./foo', './new-port');
+fs.symlink('./foo', './new-port', callback);
 ```
 
 It creates a symbolic link named "new-port" that points to "foo".
@@ -1910,7 +1910,7 @@ Example:
 ```js
 fs.writeFile('message.txt', 'Hello Node.js', (err) => {
   if (err) throw err;
-  console.log('It\'s saved!');
+  console.log('The file has been saved!');
 });
 ```