SDK: Improve the C++ SDK readme, and add some missing docstrings.

sdk_build/data/cpp/README-sdk.md

 # Mojo C++ SDK
 
 This is a minimal SDK for writing/building Mojo applications/services in C++.
 Currently, it supports building for Linux. (**TODO(vtl)**: Support other
 configurations/targets.)
 
 It includes a simple Makefile that demonstrates building a couple of simple
 example applications/services (in `examples/hello_mojo`). Note that the Makefile
 is intended to be illustrative, and not for maximal correctness or practicality
 (for example, it does not deal with dependencies correctly).
 
 Users of the SDK are expected to set up builds in an appropriate way for their
 situation, but the Makefile should show the key steps (in particular, generating
 the C++ source files from Mojom files).
 
 ## Prerequisites
 
 To fetch the Mojom tool and clang:
 * bash
 * python 2.7
 
 To build the examples:
 * binutils
 * make
 
 ## Set up
 
-To generate code from Mojom files, one needs the Mojom tool. Running the
-following causes the binary to be downloaded:
+To get started quickly, just run the following:
+
+    $ mojo_sdk_setup/setup.sh
+
+This is equivalent to performing the steps below separately:
 
     $ mojo_sdk_setup/download_mojom_tool.sh
-
-The included Makefile uses clang. The following downloads a suitable version of
-clang:
-
     $ mojo_sdk_setup/download_clang.sh
 
-(One should be able to build using another compiler with sufficient C++11
-support.)
+The first downloads a binary for the Mojom tool, which is needed to generate
+code from Mojom files. The second downloads a suitable version of clang, which
+is needed by the included Makefile. (One should also be able to build using
+another compiler with sufficient C++11 support.)
 
 ## Building the examples
 
 To build the included examples:
 
     $ make -j8
 
 (or similar). All the build output is put into the `out` subdirectory. The
 examples themselves are `hello_mojo_client.mojo` and `hello_mojo_server.mojo`.
 
 ## See also
 
 * [The Mojo project on GitHub](https://github.com/domokit/mojo)
 * [Mojo documentation](https://github.com/domokit/mojo/tree/master/docs)