RFR: 8336331: Doc: Clarification in AccessibleAttribute, AccessibleRole

[Andy Goryachev]

Minor clarifications in Javadoc

**AccessibleAttribute**:
- Point2D and Bound values uses screen coordinates. Example: `BOUNDS`, `BOUNDS_FOR_RANGE`, `OFFSET_AT_POINT`, ...
- clarified the meaning of `SELECTION_END`, `SELECTION_START`

**Accessible Role**:
- missing accessible action `SHOW_TEXT_RANGE`

This will be a minor clarification, so no CSR is required.

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Commit messages:
 - 8336331: Doc: Clarification in AccessibleAttribute, AccessibleRole

Changes: https://git.openjdk.org/jfx/pull/1512/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1512&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8336331
  Stats: 12 lines in 2 files changed: 4 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jfx/pull/1512.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1512/head:pull/1512

PR: https://git.openjdk.org/jfx/pull/1512