Clarify the use of --deps option

Author: José Valim

lib/mix/lib/mix/tasks/clean.ex

@@ -8,7 +8,8 @@ defmodule Mix.Tasks.Clean do
   Deletes generated application files.
 
   This command deletes all build artifacts for the current project.
-  Dependencies' build files are cleaned if the `--deps` option is given.
+  Dependencies' sources and build files are cleaned only if the
+  `--deps` option is given.
 
   By default this task works across all environments, unless `--only`
   is given.