RFR: 8374540: Add comment describing implementaiton choices of Math.fma [v2]

[Joe Darcy]

> Add comment describing why Math.fma uses BigDecimal.

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Respond to review feedback and add conclusion.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/29044/files
  - new: https://git.openjdk.org/jdk/pull/29044/files/309afbc2..c1dac46a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29044&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29044&range=00-01

  Stats: 7 lines in 1 file changed: 3 ins; 0 del; 4 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