Updating javadoc of Plugin, LabelAtomProperty, GlobalConfiguration with ...

...my observations from working on my plugin.

I wasn't sure if this would be better on a wiki page, but since the [Jenkins wiki extension page](https://wiki.jenkins-ci.org/display/JENKINS/Extension+points) redirects to the javadoc (which is where I was looking for information while writing my code), I thought it would be worth a PR.
Could use a reference to `hudson.Extension` which basically serves as a replacement to registering extensions.