app: give some love to documentation

Update some outdated docs, and add documentation for undocumented
pieces.
app: unexport App.Find method

This method wasn't used anywhere but the app package.
app: remove App.Unit method

This wasn't used anymore.
app: unexport SerializeEnvVars

This method is used only in the app package.
app: unexport setEnvsToApp

This method should be used only in the app package, as it is the only
package that is aware of the queue. Other packages should use
App.SetEnvs.
app: remove createBucket function

Sorry about that, I left this legacy :(
api: added TODO note to createAppHelper

Let's hope it will vanish someday.