RFR: JDK-8318476: Add resource consumption note to BigInteger and BigDecimal

[Joe Darcy]

Add informative notes to BigInteger and BigDecimal about possible running times, etc.

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Commit messages:
 - Appease jcheck.
 - JDK-8318476: Add resource consumption note to BigInteger and BigDecimal

Changes: https://git.openjdk.org/jdk/pull/16298/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16298&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318476
  Stats: 50 lines in 2 files changed: 48 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/16298.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16298/head:pull/16298

PR: https://git.openjdk.org/jdk/pull/16298