RFR: 8261862: Expand discussion of rationale for BigDecimal equals/compareTo semantics

[Joe Darcy]

I considered @stuart-marks previous suggestion during the code review of JDK-8261123 to include a more explicit discussion of why, say, different representations of 2 should not be regarded as equivalent. After contemplating several alternatives, I didn't find anything simpler than Stuart's 2/3 example so I used that as seen in the diff.

A short digression, BigDecimal supports both fixed-point style and floating-point style rounding. Floating-point rounding primarily replies on the number of precision digits, regards of their scale, while fixed-point style rounding prioritizes the scale. The number of digits of eventual output is a function of number number of digits in the inputs and the number of precision digits in a floating-point style rounding. A floating-point style rounding has a preferred scale, rather than a fixed scale based on the inputs. The fixed-point style divide method used in the example has a scale based on the dividend, allowing a relatively simple expression to show a distinction between 2.0 and 2.00.

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Commit messages:
 - 8261862: Expand discussion of rationale for BigDecimal equals/compareTo semantics

Changes: https://git.openjdk.java.net/jdk/pull/2804/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2804&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261862
  Stats: 12 lines in 1 file changed: 10 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2804.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2804/head:pull/2804

PR: https://git.openjdk.java.net/jdk/pull/2804