Clarify formatter docs, closes #15616

Author: josevalim

lib/mix/lib/mix/tasks/format.ex

@@ -46,8 +46,8 @@ defmodule Mix.Tasks.Format do
       that have their own formatting rules. Each subdirectory should have a
       `.formatter.exs` that configures how entries in that subdirectory should be
       formatted as. Configuration between `.formatter.exs` are not shared nor
-      inherited. If a `.formatter.exs` lists "lib/app" as a subdirectory, the rules
-      in `.formatter.exs` won't be available in `lib/app/.formatter.exs`.
+      inherited, except for the `:line_length` option.
+
       Note that the parent `.formatter.exs` must not specify files inside the "lib/app"
       subdirectory in its `:inputs` configuration. If this happens, the behaviour of
       which formatter configuration will be picked is unspecified.