doc: document the cache parameter for fs.realpathSync

PR-URL: https://github.com/nodejs/node/pull/4285
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/fs.markdown

@@ -634,7 +634,9 @@ Synchronous version of [`fs.read()`][]. Returns the number of `bytesRead`.
 
 ## fs.realpathSync(path[, cache])
 
-Synchronous realpath(2). Returns the resolved path.
+Synchronous realpath(2). Returns the resolved path. `cache` is an
+object literal of mapped paths that can be used to force a specific path
+resolution or avoid additional `fs.stat` calls for known real paths.
 
 ## fs.rename(oldPath, newPath, callback)