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

doc/api/fs.md

@@ -2547,14 +2547,17 @@ 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
-automatically.
-
 The `fs.readFile()` function buffers the entire file. To minimize memory costs,
 when possible prefer streaming via `fs.createReadStream()`.
 
+### 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.
+
 ## fs.readFileSync(path[, options])
 <!-- YAML
 added: v0.1.8
@@ -3540,14 +3543,32 @@ 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.
-
 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
+### 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 current position. Basically, if there are
+multiple write calls to a file descriptor and then a `writeFile()` call is
+made with the same file descriptor, the data would have been appended.
+For example, if we did something like this
+```js
+fs.writeFile(fd, 'Hello', function(err) {
+  if (err) {
+    throw err;
+  }
+  fs.writeFile(fd, 'World', function(err) {
+    if (err) {
+      throw err;
+    }
+    // At this point, file will have `HelloWorld`.
+  });
+});
+```
+
 
 ## fs.writeFileSync(file, data[, options])
 <!-- YAML
@@ -3780,6 +3801,11 @@ returned.
 
 The `FileHandle` has to support reading.
 
+**Note:** If one or more `filehandle.read()` calls are made on a file handle
+and then a `filehandle.readFile()` call is made, the data will be read from
+the current position till the end of the file. It doesn't always read from the
+beginning of the file.
+
 #### filehandle.stat([options])
 <!-- YAML
 added: v10.0.0
@@ -3942,6 +3968,11 @@ The `FileHandle` has to support writing.
 It is unsafe to use `filehandle.writeFile()` multiple times on the same file
 without waiting for the `Promise` to be resolved (or rejected).
 
+**Note:** If one or more `filehandle.write()` calls are made on a file handle
+and then a `filehandle.writeFile()` call is made, the data will be written from
+the current position till the end of the file. It doesn't always write from the
+beginning of the file.
+
 ### fsPromises.access(path[, mode])
 <!-- YAML
 added: v10.0.0