Fix all javadoc warnings

[devinrsmith]

#5 adds javadoc as part of the build process. That PR fixes all the hard javadoc errors, but there are still a lot (~100) javadoc warnings; run ./gradlew javadoc to see them.

We should fix all javadoc warnings; if we want to be pedantic, we can also add logic to fail on javadoc warnings.

[devinrsmith]

Note: debian/ubuntu disable javadoc warnings by default (even though they aren't fatal...).

See https://sources.debian.org/patches/openjdk-11/11.0.13+8-1/disable-doclint-by-default.diff/