RFR: JDK-8329557: Fix statement around MathContext.DECIMAL128 rounding

[Joe Darcy]

Happened to notice a semantic typo in the description of MathContext.DECIMAL128, use of "decimal64" where "decimal128" was intended, and added some additional information to make the related descriptions more informative.

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Commit messages:
 - JDK-8329557: Fix statement around MathContext.DECIMAL128 rounding

Changes: https://git.openjdk.org/jdk/pull/18587/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18587&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329557
  Stats: 7 lines in 1 file changed: 0 ins; 0 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/18587.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18587/head:pull/18587

PR: https://git.openjdk.org/jdk/pull/18587