RFR: 8364039: Adding implNote to DOMSignContext and DOMValidateContext on JDK-specific properties

[Weijun Wang]

Document some existing JDK-specific properties for XML Digital Signature.

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Commit messages:
 - the implNote

Changes: https://git.openjdk.org/jdk/pull/26929/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26929&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364039
  Stats: 30 lines in 2 files changed: 16 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/26929.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26929/head:pull/26929

PR: https://git.openjdk.org/jdk/pull/26929