[code-reflection] RFR: Fix javadoc warnings in JavaType hierarchy

[Maurizio Cimadamore]

As the PR title says.
Several methods in JavaType missing `@return`.

I've also tweaked the `typeVarRef` factory to just be called `typeVar` -- and also tweaked the javadoc accordingly.

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Commit messages:
 - Initial push

Changes: https://git.openjdk.org/babylon/pull/897/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=897&range=00
  Stats: 48 lines in 9 files changed: 1 ins; 6 del; 41 mod
  Patch: https://git.openjdk.org/babylon/pull/897.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/897/head:pull/897

PR: https://git.openjdk.org/babylon/pull/897