RFR: JDK-8322979: Add informative discussion to Modifier

[Joe Darcy]

Add a few apiNote concerning source-level modifiers that are not represented in java.lang.reflect.Modifier.

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Commit messages:
 - JDK-8322979: Add informative discussion to Modifier

Changes: https://git.openjdk.org/jdk/pull/17338/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17338&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322979
  Stats: 27 lines in 1 file changed: 26 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17338.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17338/head:pull/17338

PR: https://git.openjdk.org/jdk/pull/17338