Add a note about line filter for a file with multiple test cases

Signed-off-by: José Valim <[email protected]>

Author: lexmag

lib/mix/lib/mix/tasks/test.ex

@@ -100,6 +100,11 @@ defmodule Mix.Tasks.Test do
 
       mix test --only line:12 test/some/particular/file_test.exs
 
+  Note that line filter takes the closest test on or before the given line number.
+  In the case a single file contains more than one test module (test case),
+  line filter applies to every test case before the given line number, thus more
+  than one test might be taken for the run.
+
   ## Configuration
 
     * `:test_paths` - list of paths containing test files, defaults to