RFR: 8336873: BasicSplitPaneDivider:oneTouchExpandableChanged() should mention that implementation depends on "SplitPane.supportsOneTouchButtons" propertySpec clarification

[Prasanta Sadhukhan]

BasicSplitPaneDivider:oneTouchExpandableChanged() spec doesn't mention that it will create left/right button only if L&F supports SplitPane.supportsOneTouchButtons which is now added in javadoc..

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Commit messages:
 - Spec clarification

Changes: https://git.openjdk.org/jdk/pull/20289/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20289&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336873
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20289.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20289/head:pull/20289

PR: https://git.openjdk.org/jdk/pull/20289