RFR: 8321140: Add comment to note difference in Metal's JButton margins [v4]

[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]

Damon Nguyen has updated the pull request incrementally with one additional commit since the last revision:

  Update copyright year

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20482/files
  - new: https://git.openjdk.org/jdk/pull/20482/files/a16b29be..5bd6948e

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20482&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20482&range=02-03

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 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