update tools.build guide re namespace compilation

content/guides/tools_build.adoc

@@ -159,14 +159,16 @@ An example build for a compiled uberjar will look like this:
   (b/copy-dir {:src-dirs ["src" "resources"]
                :target-dir class-dir})
   (b/compile-clj {:basis basis
-                  :src-dirs ["src"]
+                  :ns-compile '[my.lib.main]
                   :class-dir class-dir})
   (b/uber {:class-dir class-dir
            :uber-file uber-file
            :basis basis
            :main 'my.lib.main}))
 ----
 
+This example directs `compile-clj` to compile the main namespace (by default source will be loaded from the basis :paths). Compilation is transitive and all namespaces loaded by the compiled namespace will also be compiled. You may need to add additional namespaces if code is dynamically or optionally loaded.
+
 The deps.edn and build execution will look the same as the prior example.
 
 You can create the uber jar build with: