update docstrings about watchman_dir
update docstrings about watchman_dir

Summary:
The `getFullPath` returns an absolute path, not a path relative to watch root.
Update comments to match the actual implementation.

Reviewed By: chadaustin

Differential Revision: D37290755

fbshipit-source-id: 2a83780d8ca6559f51f27962ead982051e117358

facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman,facebook/watchman