RFR: 8340721: Clarify special case handling of unboxedType and getWildcardType

[Joe Darcy]

First piece of a larger set of updates that may be made for JDK-8055219 to clarify the exceptional behavior of `javax.lang.model.util.Types`.

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Commit messages:
 - JDK-8340721: Clarify special case handling of unboxedType and getWildcardType

Changes: https://git.openjdk.org/jdk/pull/21193/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21193&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340721
  Stats: 196 lines in 2 files changed: 194 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21193.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21193/head:pull/21193

PR: https://git.openjdk.org/jdk/pull/21193