Clarify behavior of 'close()' in the docs

[etienne428]

I am using secret-stack for an app and it took me a very long time to figure out how to call app.close(). Documentation says it can be called without argument or with '(err, callback)', but that didn't work well, and looking at the source code wasn't helpful.
In the server test (

secret-stack/test/server.js

Line 64 in 755a638

alice.close(true); bob.close(true)

), one calls 'alice.close(true)', which seem to be the way to go.

secret-stack/README.md

Line 117 in 755a638

Optionally takes `(err, callback)`