RFR: 8357063: Document preconditions for DecimalDigits methods [v2]

[Shaojin Wen]

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

Shaojin Wen has updated the pull request incrementally with two additional commits since the last revision:

 - assert
 - document add warnings

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Changes:
  - all: https://git.openjdk.org/jdk/pull/25246/files
  - new: https://git.openjdk.org/jdk/pull/25246/files/66f71c58..ddb44aa8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=25246&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=25246&range=00-01

  Stats: 29 lines in 1 file changed: 24 ins; 5 del; 0 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