tidy: replace make check with ./x.py test in documentation

CONTRIBUTING.md

@@ -142,7 +142,7 @@ the issue in question.
 Please make sure your pull request is in compliance with Rust's style
 guidelines by running
 
-    $ python x.py test src/tools/tidy
+    $ python x.py test tidy
 
 Make this check before every pull request (and every new commit in a pull
 request); you can add [git hooks](https://git-scm.com/book/en/v2/Customizing-Git-Git-Hooks)

src/bootstrap/README.md

@@ -55,6 +55,9 @@ The script accepts commands, flags, and arguments to determine what to do:
   # run all unit tests
   ./x.py test
 
+  # execute tool tests
+  ./x.py test tidy
+
   # execute the UI test suite
   ./x.py test src/test/ui
 

src/bootstrap/flags.rs

@@ -359,7 +359,7 @@ 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
@@ -372,6 +372,10 @@ Arguments:
     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.
 

src/tools/tidy/src/main.rs

@@ -1,8 +1,8 @@
 //! Tidy checks source code in this repository.
 //!
 //! This program runs all of the various tidy checks for style, cleanliness,
-//! etc. This is run by default on `make check` and as part of the auto
-//! builders.
+//! etc. This is run by default on `./x.py test` and as part of the auto
+//! builders. The tidy checks can be executed with `./x.py test tidy`.
 
 #![deny(warnings)]