RFR: 4668290: unclear spec for Polygon.bounds field

[SWinxy]

New documentation replaces the `This value can be null.`:

When created, {@link #invalidate() invalidated}, or {@link #reset() reset},
this becomes {@code null}. To get out of the null state,
{@link #getBounds()} called with {@link #npoints} being greater than
{@code 0} will transfer into a non-null {@link Rectangle}.


I think I've got the reason for why it can be `null` correct, but you never know.

In javax.swing.text.html.Map, I've replaced setting the field to null with the equivalent #invalidate(). This may mean we can make the field private in the future.

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Commit messages:
 - 4668290: unclear spec for Polygon.bounds field

Changes: https://git.openjdk.org/jdk/pull/10417/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10417&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-4668290
  Stats: 7 lines in 2 files changed: 4 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/10417.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10417/head:pull/10417

PR: https://git.openjdk.org/jdk/pull/10417