api: note get_url doesn't chekc file existence

per https://github.com/iterative/dvc/pull/3130#discussion_r367359439

dvc/api.py

@@ -58,6 +58,7 @@ def get_url(path, repo=None, rev=None, remote=None):
     """
     Returns the full URL to the data artifact specified by its `path` in a
     `repo`.
+    NOTE: There is no guarantee that the file actually exists in that location.
     """
     try:
         with _make_repo(repo, rev=rev) as _repo: