RFR: 8321140: Doc change to note difference in Metal's JButton margins

[Damon Nguyen]

Previously in [JDK-8282772](https://bugs.openjdk.org/browse/JDK-8282772), there was a fix for JButtons with HTML content alignment for non-MacOS L&Fs. This fix was to make other L&Fs in line with the Aqua fix found in [JDK-8015854](https://bugs.openjdk.org/browse/JDK-8015854).

However, the non-MacOS L&F fix caused a regression and the details and original discussion can be found in [JDK-8318590](https://bugs.openjdk.org/browse/JDK-8318590). This non-MacOS L&F fix was backed out and the current behavior for JButtons with HTML content for these L&Fs was deemed to be correct as is.

Through discussion, a note about Metal's different margin values should be added somewhere for future reference. Metal's horizontal margins are 14 where other L&Fs are zero or some small value. This discrepancy is what caused the original issue to be filed. This change is to add a comment to `MetalLookAndFeel.java` since there is no exact spot where these values are set for the margin. It's been added where other Metal components (CheckBox, RadioButton, etc.) have comments on margins as well.

Here are some L&F values for reference/comparison:

Metal's margins: [top=2,left=14,bottom=2,right=14]
Aqua's margins: [top=0,left=2,bottom=0,right=2]
Motif's margins: [top=2,left=4,bottom=2,right=4]
Nimbus's margins: [top=0,left=0,bottom=0,right=0]

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/20482/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20482&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321140
  Stats: 48 lines in 1 file changed: 36 ins; 9 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/20482.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20482/head:pull/20482

PR: https://git.openjdk.org/jdk/pull/20482