RFR: 8327167: Add add() example for Leap Day in Calendar's doc

[Naoto Sato]

A simple doc update to include a leap day example in the `Calendar` class.

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Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/18158/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18158&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327167
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/18158.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18158/head:pull/18158

PR: https://git.openjdk.org/jdk/pull/18158