RFR: 8261366: Add discussion of IEEE 754 to BigDecimal

[Joe Darcy]

Informative update to BigDecimal and related classes to latest IEEE 754 terminology, including a discussion of the similarities and differences of BigDecimal and IEEE 754 decimal arithmetic.

Once the wording is finalized, I'll reflow any modified paragraphs and update the copyright years, etc.

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Commit messages:
 - 8261366: Add discussion of IEEE 754 to BigDecimal

Changes: https://git.openjdk.java.net/jdk/pull/2868/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2868&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261366
  Stats: 88 lines in 3 files changed: 60 ins; 3 del; 25 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2868.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2868/head:pull/2868

PR: https://git.openjdk.java.net/jdk/pull/2868