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.
Isn't this already part of the *Extend Jenkins* part of the wiki? If not, it should be. Much of this is general advice applicable to a bunch of extension points, so documenting this here just reduces visibility IMO.