Fix javadoc warnings in generated files

[ST-DDT]

Currently the protoc generated java classes will cause javadoc compilation warnings due to missing @param and @return values.

I first thought this was an easy change, but I soon discovered that this takes quite some effort.
That's why I would like to get some feedback whether these changes have a chance to be merged at all / are appreciated.

This PR is by no means complete, I just changed about 50/430 occurances of WriteFieldDocComment.

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[ST-DDT]

I signed it!

[googlebot]

CLAs look good, thanks!

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[ghost]

This generally looks good to me. Thank you for doing this! Would you be willing to follow up with more changes to update the remaining WriteFieldDocComment?

[ST-DDT]

That's the plan. It might take some time, though, as I currently have only limited free time.

[ST-DDT]

Progress: 255/450 (55%)

[ghost]

This is looking very good. Let's hold at this state for now. I want to make sure this can successfully get in (the larger the PR, the harder it is to get in).

[ST-DDT]

Currently it is too hot to concentrate properly. I'll wait with the fixes until it gets a little bit colder.

[ST-DDT]

I have fixed the above mentioned issues.

[ghost]

This looks good. Thank you for improving protobuf!