RFR: 8306320: Add @implSpec for explaining BufferedImage default behavior

[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.

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Commit messages:
 - Added @implSpec for explaining the default behavior, which is quite different from what the javadoc said.

Changes: https://git.openjdk.org/jdk/pull/13506/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13506&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8306320
  Stats: 36 lines in 1 file changed: 33 ins; 0 del; 3 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