better comment

repo2docker/buildpacks/repo2docker-entrypoint

@@ -24,8 +24,11 @@ SIGNALS = set(signal.Signals) - {signal.SIGKILL, signal.SIGSTOP, signal.SIGCHLD}
 
 def main():
 
-    # open log file to send output; iterate through different
-    # base locations
+    # open log file to send output to;
+    # preferred location of log file is:
+    #  1. REPO_DIR env variable
+    #  2. current working directory: "."
+    #  3. default temp directory for the OS (e.g. /tmp for linux)
     log_dirs = [".", tempfile.gettempdir()]
     if "REPO_DIR" in os.environ:
         log_dirs.insert(0, os.environ["REPO_DIR"])