Fix remaining Javadoc gaps that hide meaningful doclint warnings

A number of public members in `core/` have Javadoc comments with no summary and no `@return` tag, so #45's `{@return ...}` conversion doesn't apply to them. Javadoc uses the first sentence as the summary shown in the member tables, so these currently show up blank.

I want to add a short written summary to each of them. Many won't say much beyond the name, but filling them in means `-Xdoclint` can be turned back on for `core` and to expose warnings that actually matter.

I've already made the changes and plan to open a PR.

As before, please let me know if you don't want changes like these. I'll stop posting if you don't want them.

Review the public members in core whose Javadoc lacks a first-sentence summary or @return tag, and compare the intended pattern with #45's {@return ...} conversion. Done means each affected member has a short summary and core can be checked with -Xdoclint to expose meaningful warnings.

Written by the indexing model from the issue text.