doc: move fs.existsSync() deprecation message

This commit moves the deprecation message for fs.existsSync()
above the function description, making message placement
uniform across the documentation.

PR-URL: #3942
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/fs.markdown

@@ -364,11 +364,11 @@ non-existent.
 
 ## fs.existsSync(path)
 
+    Stability: 0 - Deprecated: Use [fs.statSync][] or [fs.accessSync][] instead.
+
 Synchronous version of [`fs.exists`][].
 Returns `true` if the file exists, `false` otherwise.
 
-    Stability: 0 - Deprecated: Use [fs.statSync][] or [fs.accessSync][] instead.
-
 ## fs.fchmod(fd, mode, callback)
 
 Asynchronous fchmod(2). No arguments other than a possible exception