Add help text for the plugin

[buep]

It would be nice if the plugin had help text, and its name linked to each as it seems to be common for other plugins.

See the picture, where clicking on the blue questionmark icon open a help text for that specific input field. It also states which plugin contributes to the user interface here.

We will like that for memory map as well.

Originally reported as https://issues.jenkins-ci.org/browse/JENKINS-19164 (closed as we only use github issues now)