doc: hello_world: touch up README

Since this is such a basic sample, it's worth nitpicking the wording a
bit:

- Add a link to the supported boards, since many first time users
  will not know where it is and may start near here.

- Remove the single and multi threaded note. There's no information on
  how to build it in either of these two modes or what the default is,
  so it feels like a distraction for such a basic sample.

- Give a clue about how to build for another board.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>

samples/hello_world/README.rst

@@ -5,17 +5,14 @@ Hello World
 
 Overview
 ********
-A simple Hello World example that can be used with any supported board and
-prints 'Hello World' to the console. This application can be built into modes:
 
-* single thread
-* multi threading
+A simple sample that can be used with any :ref:`supported board <boards>` and
+prints "Hello World" to the console.
 
 Building and Running
 ********************
 
-This project outputs 'Hello World' to the console.  It can be built and executed
-on QEMU as follows:
+This application can be built and executed on QEMU as follows:
 
 .. zephyr-app-commands::
    :zephyr-app: samples/hello_world
@@ -24,6 +21,8 @@ on QEMU as follows:
    :goals: run
    :compact:
 
+To build for another board, change "qemu_x86" above to that board's name.
+
 Sample Output
 =============