RFR: 6391806: JLabel and AbstractButton's imageUpdate method should be better specified

[Abhishek Kumar]

JLabel and AbstractButton's imageUpdate method description updated.

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### Progress
- [ ] Change must be properly reviewed (1 review required, with at least 1 [Reviewer](https://openjdk.org/bylaws#reviewer))
- [x] Change must not contain extraneous whitespace
- [x] Commit message must refer to an issue

### Error
 ⚠️ OCA signatory status must be verified

### Integration blocker
 ⚠️ Title mismatch between PR and JBS for issue [JDK-6391806](https://bugs.openjdk.org/browse/JDK-6391806)



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Commit messages:
 - imageUpdate method description changed

Changes: https://git.openjdk.org/jdk/pull/9240/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9240&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-6391806
  Stats: 6 lines in 2 files changed: 1 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/9240.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9240/head:pull/9240

PR: https://git.openjdk.org/jdk/pull/9240