RFR: 8364146: JList getScrollableUnitIncrement return 0

[Prasanta Sadhukhan]

It seems JList.getScrollableUnitIncrement can sometime return 0 instead of positive number which is not specified in the javadoc which can lead to confusion. Clarified javadoc as to when it can return 0.

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Commit messages:
 - 8364146: JList getScrollableUnitIncrement return 0

Changes: https://git.openjdk.org/jdk/pull/26500/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26500&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8364146
  Stats: 5 lines in 1 file changed: 3 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/26500.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26500/head:pull/26500

PR: https://git.openjdk.org/jdk/pull/26500