RFR: 8374540: Add comment describing implementaiton choices of Math.fma

[Joe Darcy]

Add comment describing why Math.fma uses BigDecimal.

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Commit messages:
 - JDK-8374540: Add comment describing implementaiton choices of Math.fma

Changes: https://git.openjdk.org/jdk/pull/29044/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29044&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374540
  Stats: 13 lines in 1 file changed: 12 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/29044.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29044/head:pull/29044

PR: https://git.openjdk.org/jdk/pull/29044