Add documentation of tool testing with x.py script

src/bootstrap/flags.rs

@@ -359,19 +359,23 @@ Arguments:
                 subcommand_help.push_str(
                     "\n
 Arguments:
-    This subcommand accepts a number of paths to directories to tests that
+    This subcommand accepts a number of paths to test directories that
     should be compiled and run. For example:
 
         ./x.py test src/test/ui
         ./x.py test src/libstd --test-args hash_map
         ./x.py test src/libstd --stage 0 --no-doc
         ./x.py test src/test/ui --bless
         ./x.py test src/test/ui --compare-mode nll
-
+    
     Note that `test src/test/* --stage N` does NOT depend on `build src/rustc --stage N`;
     just like `build src/libstd --stage N` it tests the compiler produced by the previous
     stage.
 
+    Execute tool tests with a tool name argument:
+
+        ./x.py test tidy
+
     If no arguments are passed then the complete artifacts for that stage are
     compiled and tested.