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

sdk_build/data/cpp/README-sdk.md

Index: sdk_build/data/cpp/README-sdk.md
diff --git a/sdk_build/data/cpp/README-sdk.md b/sdk_build/data/cpp/README-sdk.md
index be620c7759b6c96807bce6190a9da70835c3b347..6144fa82d9e29bd76da33d01888dd087e3243a11 100644
--- a/sdk_build/data/cpp/README-sdk.md
+++ b/sdk_build/data/cpp/README-sdk.md
@@ -25,18 +25,19 @@ To build the examples:
 
 ## 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/download_mojom_tool.sh
+    $ mojo_sdk_setup/setup.sh
 
-The included Makefile uses clang. The following downloads a suitable version of
-clang:
+This is equivalent to performing the steps below separately:
 
+    $ mojo_sdk_setup/download_mojom_tool.sh
     $ 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