RFR: 8318590: JButton ignores margin when painting HTML text

[Damon Nguyen]

The initial issue was a padding issue for JButton when HTML content was set as the button's text. [JDK-8282772](https://bugs.openjdk.org/browse/JDK-8282772), [JDK-8015854](https://bugs.openjdk.org/browse/JDK-8015854), and [JDK-8302173](https://bugs.openjdk.org/browse/JDK-8302173) are the issues related to the fix. The initial bug report was for Metal L&F, since it has a drastically different button margin from the rest (2, 14, 2, 14). This left and right margin of 14px led to the initial bug report. However, it was determined that the issue was not an issue, and instead, a problem with the reporter's implementation since they weren't aware of the difference in Metal's button margins versus other L&Fs.

This PR reverts these changes since it was determined in [this PR](https://github.com/JFormDesigner/FlatLaf/issues/746) that the fix causes a new issue regarding JButton contents' positioning. In addition, a new line is added to BasicLookAndFeel (since Metal derives its default button margin here) to better document the difference in button margins compared to other L&Fs.

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Commit messages:
 - Initial fix. Revert changes and update doc instead.

Changes: https://git.openjdk.org/jdk/pull/16869/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16869&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318590
  Stats: 195 lines in 5 files changed: 11 ins; 177 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/16869.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16869/head:pull/16869

PR: https://git.openjdk.org/jdk/pull/16869