doc: implicit fs callbacks don't throw in v0.10

But they will in v0.12.

Re #5005.

doc/api/fs.markdown

@@ -63,26 +63,11 @@ Relative path to filename can be used, remember however that this path will be
 relative to `process.cwd()`.
 
 Most fs functions let you omit the callback argument. If you do, a default
-callback is used that rethrows errors. To get a trace to the original call
-site, set the NODE_DEBUG environment variable:
+callback is used that ignores errors, but prints a deprecation
+warning.
 
-    $ cat script.js
-    function bad() {
-      require('fs').readFile('/');
-    }
-    bad();
-
-    $ env NODE_DEBUG=fs node script.js
-    fs.js:66
-            throw err;
-                  ^
-    Error: EISDIR, read
-        at rethrow (fs.js:61:21)
-        at maybeCallback (fs.js:79:42)
-        at Object.fs.readFile (fs.js:153:18)
-        at bad (/path/to/script.js:2:17)
-        at Object.<anonymous> (/path/to/script.js:5:1)
-        <etc.>
+**IMPORTANT**: Omitting the callback is deprecated.  v0.12 will throw the
+errors as exceptions.
 
 
 ## fs.rename(oldPath, newPath, [callback])