Add javadoc for generated constructors

[Rawi01]

This PR fixes #933

Lombok now adds a javadoc comment to a generated constructor if there is any field with a description.

[archer-321]

Thanks for working on this feature!

Would it make sense to allow a -- CONSTRUCTOR -- section similar to -- GETTER --? This wouldn't work for classes with multiple constructor annotations unless the syntax is changed to -- ALL CONSTRUCTOR --, -- REQUIRED CONSTRUCTOR -- or similar, but it would allow for more flexible constructor documentation.

Looking at the example generated documentation in the tests, I immediately noticed the capitalization for the @param parameter documentation. Most projects I'm aware of (including the OpenJDK source code) use lowercase descriptions while starting class item documentation with a cardinal letter. More control over the generated constructor documentation would help keep the module documentation uniform.

[Rawi01]

@archer-321 Thanks for the feedback. The intended solution was to copy @param tags to the constructor if they are present. I missed that it doesn't work at all because of different bug that I fixed years ago but was never merged. Cherry picked that commit and it should now work as planned.

[rzwitserloot]

Speaking of javadoc, @Rawi01 there's the markdown JEP 467 - the only effect I think that has on lombok is that we should copy the 'this is markdown style' flag to the javadoc node we generate, other than that, I don't think this affects either scanning for --- GETTER --- or scanning for @param / @return, which is unchanged in the markdown flavour. We can only move on this once all relevant tools (javac and ecj I guess) are updated to be capable of handling JEP 467 which probably isn't yet today? I haven't checked.

This does not affect accepting this PR at all, just tangentially related.