RFR: 8250659: Clarify in ParameterizedType.getRawType() doc that only Class is returned

[Chen Liang]

Clarify that only `Class` is returned for core reflection implementation, and the return type is `Type` to support other implementations, such as ones modeling unloaded or remote types.

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Commit messages:
 - 8250659: Clarify in ParameterizedType.getRawType() doc that only Class is returned

Changes: https://git.openjdk.org/jdk/pull/19978/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19978&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8250659
  Stats: 12 lines in 1 file changed: 7 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/19978.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19978/head:pull/19978

PR: https://git.openjdk.org/jdk/pull/19978