doc: clarify fd closing by `fs.readFile` etc.

Ref: https://github.com/nodejs/node/issues/7560
PR-URL: https://github.com/nodejs/node/pull/7561
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/fs.md

@@ -384,7 +384,8 @@ fs.appendFile('message.txt', 'data to append', 'utf8', callback);
 
 Any specified file descriptor has to have been opened for appending.
 
-_Note: Specified file descriptors will not be closed automatically._
+_Note: If a file descriptor is specified as the `file`, it will not be closed
+automatically._
 
 ## fs.appendFileSync(file, data[, options])
 <!-- YAML
@@ -1136,7 +1137,8 @@ fs.readFile('/etc/passwd', 'utf8', callback);
 
 Any specified file descriptor has to support reading.
 
-_Note: Specified file descriptors will not be closed automatically._
+_Note: If a file descriptor is specified as the `file`, it will not be closed
+automatically._
 
 ## fs.readFileSync(file[, options])
 <!-- YAML
@@ -1671,7 +1673,8 @@ Note that it is unsafe to use `fs.writeFile` multiple times on the same file
 without waiting for the callback. For this scenario,
 `fs.createWriteStream` is strongly recommended.
 
-_Note: Specified file descriptors will not be closed automatically._
+_Note: If a file descriptor is specified as the `file`, it will not be closed
+automatically._
 
 ## fs.writeFileSync(file, data[, options])
 <!-- YAML