help: update usage for commands that use the targets argument

dvc/command/commit.py

@@ -65,6 +65,10 @@ def add_parser(subparsers, parent_parser):
         help="Commit cache for subdirectories of the specified directory.",
     )
     commit_parser.add_argument(
-        "targets", nargs="*", default=None, help="DVC-files."
+        "targets",
+        nargs="*",
+        default=None,
+        help="DVC-files to commit. Optional."
+        "(Finds all DVC-files in the workspace by default.)",
     )
     commit_parser.set_defaults(func=CmdCommit)