doc: improve fs.open() docs

doc/api/fs.markdown

@@ -270,16 +270,18 @@ An exception occurs if the file does not exist.
 * `'r+'` - Open file for reading and writing.
 An exception occurs if the file does not exist.
 
-* `'rs'` - Open file for reading, telling the OS to open it synchronously
-(ie using the O_SYNC flag).  Whilst rarely useful, when used with caution by
-those who know what they're doing it can be sometimes necessary.  Note that
-this doesn't turn `fs.open()` into a synchronous blocking call, if that's what
-you want then you should be using `fs.openSync()`
-An exception occurs if the file does not exist.
+* `'rs'` - Open file for reading in synchronous mode. Instructs the operating
+  system to bypass the local file system cache.
+
+  This is primarily useful for opening files on NFS mounts as it allows you to
+  skip the potentially stale local cache. It has a very real impact on I/O
+  performance so don't use this mode unless you need it.
+
+  Note that this doesn't turn `fs.open()` into a synchronous blocking call.
+  If that's what you want then you should be using `fs.openSync()`
 
 * `'rs+'` - Open file for reading and writing, telling the OS to open it
-synchronously.
-See notes for `'rs'` about using this with caution.
+  synchronously. See notes for `'rs'` about using this with caution.
 
 * `'w'` - Open file for writing.
 The file is created (if it does not exist) or truncated (if it exists).