Add more documentation

[sco0ter]

I was reading through https://spotbugs.github.io/spotbugs-maven-plugin/check-mojo.html
and it lacks a lot of documentation.

Most or all properties have (no description).

As a developer I want to know what all these properties do.
E.g. I tried <relaxed>true</relaxed> and noticed that there is more output, i.e. bug analysis is more strict (although it contradicts a relaxed behavior somewhat).

[hazendaz]

@sco0ter We have added groovydocs to the documentation now which does show what the values are. It is unclear to the team right now why running normal build does not pick up those values and in some cases defines items that are not what we have defined. We suspect since the plugin is entirely written in groovy that groovy with maven behaves differently. At the moment no ETA on a better fix.

[hazendaz]

Maven does not support this still. See https://github.com/search?q=repo%3Aapache%2Fmaven-plugin-tools+groovy&type=pullrequests

[hazendaz]

Also see apache/maven-plugin-tools#152