RFR: 8357063: Document preconditions for DecimalDigits methods

[Shaojin Wen]

Similar to PR #24982 
Document preconditions on certain DecimalDigits methods that use operations either unsafe and/or without range checks.

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Commit messages:
 - Document preconditions for DecimalDigits methods

Changes: https://git.openjdk.org/jdk/pull/25246/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25246&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357063
  Stats: 53 lines in 7 files changed: 0 ins; 0 del; 53 mod
  Patch: https://git.openjdk.org/jdk/pull/25246.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25246/head:pull/25246

PR: https://git.openjdk.org/jdk/pull/25246