Add missing Javadoc comments to types/classes.

[blackwinter]

Serves in preparation for raising the minimum Java version to 21 (#765).

JDK 15 and later introduce additional checks in the -Xdoclint option (which appears to be enabled by default in Gradle's javadoc task), requiring Javadoc for public and protected (and even some private!?) types and members. In order to avoid this hassle (for the time being), we disable DocLint's missing check and instead rely on Checkstyle's MissingJavadocType check (as well as MissingJavadocMethod, introduced in #396). This only applies to public scope, though.

An alternative approach would be to individually suppress the warnings right where they're originating from. This would still be quite a pain, but it would make it explicit where documentation is missing (according to DocLint).

The documentation added here is intentionally curt. If you feel strongly about expanding some of it, please add concrete suggestions. Otherwise, we can deal with it later (maybe even transfer the Fix command documentation from the README to the individual classes and generate the Markdown from those).

Future steps:

• "warning: no main description" (JDK-8272374: doclint should report missing "body" comments)
• "warning: use of default constructor, which does not provide a comment" (JDK-8249634: doclint should report implicit constructor as missing javadoc comments)
• More Javadoc checks

[TobiasNx]

+1!