Update documentation for eclipse jdt in plugin-maven

[PrivateerPrime]

Since maven-plugin 2.40.0 it is possible to set up p2 mirrors for eclipse jdt. The documentation for maven plugin does not reflect that. An example can copied from #1821

 <eclipse>
 …
 <p2Mirrors>
 <p2Mirror>
 <prefix>https://download.eclipse.org/</prefix>
 <url>https://some.internal.mirror/eclipse</url>
 </p2Mirror>
 </p2Mirrors>
 </eclipse>

[nedtwigg]

PRs welcome :)