Tweak the usage messages for x.py build and x.py check.

They're a bit out of date, and overly complicated.

Author: nnethercote

src/bootstrap/flags.rs

@@ -401,26 +401,19 @@ To learn more about a subcommand, run `./x.py <subcommand> -h`",
                     "\n
 Arguments:
     This subcommand accepts a number of paths to directories to the crates
-    and/or artifacts to compile. For example:
-
-        ./x.py build library/core
-        ./x.py build library/core library/proc_macro
-        ./x.py build library/std --stage 1
-
-    If no arguments are passed then the complete artifacts for that stage are
-    also compiled.
+    and/or artifacts to compile. For example, for a quick build of a usable
+    compiler:
 
-        ./x.py build
-        ./x.py build --stage 1
+        ./x.py build --stage 1 library/std
 
-    For a quick build of a usable compiler, you can pass:
+    This will build a compiler and standard library from the local source code.
+    Once this is done, build/$ARCH/stage1 contains a usable compiler.
 
-        ./x.py build --stage 1 library/test
+    If no arguments are passed then the default artifacts for that stage are
+    compiled. For example:
 
-    This will first build everything once (like `--stage 0` without further
-    arguments would), and then use the compiler built in stage 0 to build
-    library/test and its dependencies.
-    Once this is done, build/$ARCH/stage1 contains a usable compiler.",
+        ./x.py build --stage 0
+        ./x.py build ",
                 );
             }
             "check" | "c" => {
@@ -430,14 +423,9 @@ Arguments:
     This subcommand accepts a number of paths to directories to the crates
     and/or artifacts to compile. For example:
 
-        ./x.py check library/core
-        ./x.py check library/core library/proc_macro
+        ./x.py check library/std
 
-    If no arguments are passed then the complete artifacts are compiled: std, test, and rustc. Note
-    also that since we use `cargo check`, by default this will automatically enable incremental
-    compilation, so there's no need to pass it separately, though it won't hurt. We also completely
-    ignore the stage passed, as there's no way to compile in non-stage 0 without actually building
-    the compiler.",
+    If no arguments are passed then many artifacts are checked.",
                 );
             }
             "clippy" => {