Fix #1737: Specify toolchain part in building and running

There was an issue raised that the toolchain explanation for building
and running the compiler needed some refinement. This patch aims to
remedy that by specifying what "first" and "second" refer to, and what
the toolchain does. Small change to the language to specify that the
stage2 toolchain is not built, and how one would build it.

Author: Mattias-Petersson

src/building/how-to-build-and-run.md

@@ -277,18 +277,20 @@ default).
 Once you have successfully built `rustc`, you will have created a bunch
 of files in your `build` directory. In order to actually run the
 resulting `rustc`, we recommend creating rustup toolchains. The first
-one will run the stage1 compiler (which we built above). The second
-will execute the stage2 compiler (which we did not build, but which
-you will likely need to build at some point; for example, if you want
-to run the entire test suite).
+command listed below runs the stage1 compiler, which was built in the
+steps above, with the name `stage1`. The second command runs the stage2 
+compiler using the stage1 compiler. This will be needed in the future 
+if running the entire test suite, but will not be built in this page. 
+Building stage2 is done with the same `./x build` command as for stage1,
+specifying that the stage is 2 instead.
 
 ```bash
 rustup toolchain link stage1 build/host/stage1
 rustup toolchain link stage2 build/host/stage2
 ```
 
-Now you can run the `rustc` you built with. If you run with `-vV`, you
-should see a version number ending in `-dev`, indicating a build from
+Now you can run the `rustc` you built with via the toolchain. If you run with 
+`-vV`, you should see a version number ending in `-dev`, indicating a build from
 your local environment:
 
 ```bash