Clarify Maven configuration

[anthonyvdotbe]

The current way of using .mvn/jvm.config only works if the maven-compiler-plugin doesn't use an external executable.
The previous way of using compilerArgs only works if the maven-compiler-plugin does use an external executable.

So both ways should be documented, such that users can select the way that works for their Maven setup.
Also note that setting fork to true is just one way to get the maven-compiler-plugin to use an external executable. Enabling the maven-toolchains-plugin is another.

PS: not sure whether I should create an issue for this, or if this can go under #2786?

[cushon]

Thanks! Merged as 175b9db