rebar3 hex docs: Improve output when generating custom docs

[wojtekmach]

Currently we have this:

% rebar3 hex docs
===> Verifying dependencies...
===> No valid hex docs configuration found. Docs will will not be generated
Local Password: ****

The ===> No valid hex docs ... piece is in red. This is a non-fatal error and the process moves forward (as you can see by the password prompt). Since it's non-fatal, should we change the colour to yellow or something? Better yet, would it make sense to mention that since we're not generating docs, we're instead using docs from doc/ (or whatever is configured)?