RFR: 8306320: BufferedImage spec needs clarification w.r.t its implementation of the WritableRenderedImage interface [v2]

[Martin Desruisseaux]

> `BufferedImage` implements the `WritableRenderedImage` interface. But the Javadoc is copied from `WritableRenderedImage`, while `BufferedImage` does something quite different. In particular, `TileObserver` are ignored. This pull request add `@implSlec` for explaining the default behaviour.
> 
> This commit has one specification change in `isTileWritable`: the exception type is changed from `ArrayIndexOutOfBoundsException` to `IllegalArgumentException` for matching the implementation. The logical conditions is also corrected.
> 
> This commit contains a trivial code change: `new Point(0,0)` is replaced by `new Point()` for saving a few byte codes.

Martin Desruisseaux has updated the pull request incrementally with one additional commit since the last revision:

  Update documentation for adressing comment on pull requests, with two changes to be discussed:
  
  - The "The default implementation" sentence has not yet been removed, for reason discussed on the pull request.
  - The discussion about (0,0) tile indices mentions the relationship with `getTileMinX()` and `getTileMinY()`.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13506/files
  - new: https://git.openjdk.org/jdk/pull/13506/files/2bb3a5ce..0f339543

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13506&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13506&range=00-01

  Stats: 24 lines in 1 file changed: 4 ins; 4 del; 16 mod
  Patch: https://git.openjdk.org/jdk/pull/13506.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13506/head:pull/13506

PR: https://git.openjdk.org/jdk/pull/13506