update build help text

tools/help.txt

@@ -233,24 +233,26 @@ Deprecated command. Use 'build' instead.
 Deprecated command. Use 'build' instead.
 
 >>> build
-Outputs builds for all targeted platforms.
+Build this project for all platforms.
 Usage: meteor build <output path> [--debug] [--directory]
           [--server http://example.com:3000]
 
-Package this project up for deployment. The output is a directory with several
-build artifacts:
+Package this project up for deployment. The command outputs a directory with
+builds for all platforms in this project.
 
-- a tarball that includes everything necessary to run the application server
-  (see README in the tarball for details)
-- an unassigned apk bundle and a project source if Android is targeted as a
-  mobile platform
-- a directory with an Xcode project source if iOS is targeted as a mobile
-  platform
+By default, the output directory will contain just a tarball that includes
+everything necessary to run the application server. (See README in the tarball
+for details.)
+
+If you have added mobile platforms to your project with the
+'meteor add-platform' command, then the output directory will contain
+subdirectories named 'android' (with the APK bundle and Android project
+source) and/or 'ios' (with the Xcode project source).
 
 
 Options:
-  --debug          build in debug mode (don't minify, etc)
-  --directory      output a directory (rather than a tarball) for the
+  --debug          Build in debug mode (don't minify, etc).
+  --directory      Output a directory (rather than a tarball) for the
                    application server bundle. If the output location exists,
                    it will be recursively deleted first.
   --server         Location where mobile builds connect to the Meteor server.