Warn users to encrypt their notification API tokens

Many users will search the docs and jump straight to the Notifications page to figure out how to hook up Campfire/HipChat/Flowdock etc. Many of them will not think to take the additional step to read about the encryption options available. There is plenty evidence of this in the field...

This change is similar to https://github.com/travis-ci/travis-ci.github.com/pull/308 but goes a little further by adding an explicit notice in each section that suggests using a private API token, and providing an example command line. I felt it was important to repeat the language in each section, as users are likely to only read the section relevant to the service they care about.