doc: clarify fd behaviour with {read,write}File

Author: thefourtheye

doc/api/fs.md

@@ -2547,10 +2547,13 @@ fs.readFile('<directory>', (err, data) => {
 });
 ```
 
-Any specified file descriptor has to support reading.
-
-If a file descriptor is specified as the `path`, it will not be closed
+### File Descriptors
+1. Any specified file descriptor has to support reading.
+2. If a file descriptor is specified as the `path`, it will not be closed
 automatically.
+3. The reading will begin at the current position. If the file size is
+10 bytes and if six bytes are already read with this file descriptor, then
+`readFile` will return only the rest of the four bytes.
 
 The `fs.readFile()` function buffers the entire file. To minimize memory costs,
 when possible prefer streaming via `fs.createReadStream()`.
@@ -3540,14 +3543,21 @@ If `options` is a string, then it specifies the encoding:
 fs.writeFile('message.txt', 'Hello Node.js', 'utf8', callback);
 ```
 
-Any specified file descriptor has to support writing.
+### File Descriptors
+1. Any specified file descriptor has to support writing.
+2. If a file descriptor is specified as the `file`, it will not be closed
+automatically.
+3. The writing will begin at the beginning of the file. If the file size
+is 10 bytes and if six bytes are written with this file descriptor, then
+`writeFile` will return six bytes newly written and four bytes from the file.
+For example, if the file already had `'Hello World'` and the newly written
+content is `'Aloha'`, then the contents of the file would be `'Aloha World'`,
+rather than just `'Aloha'`.
 
 It is unsafe to use `fs.writeFile()` multiple times on the same file without
 waiting for the callback. For this scenario, [`fs.createWriteStream()`][] is
 recommended.
 
-If a file descriptor is specified as the `file`, it will not be closed
-automatically.
 
 ## fs.writeFileSync(file, data[, options])
 <!-- YAML