RFR: 8374540: Add comment describing implementaiton choices of Math.fma [v3]

[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:

  Add note on interpreter.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/29044/files
  - new: https://git.openjdk.org/jdk/pull/29044/files/c1dac46a..c04963c0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29044&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29044&range=01-02

  Stats: 5 lines in 1 file changed: 1 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