RFR: 8375243: Improve javafx.scene.layout package documentation

[Michael Strauß]

This PR adds a paragraph about layout orientation to the `javafx.scene.layout` package documentation.

-------------

Commit messages:
 - javafx.scene.layout package documentation

Changes: https://git.openjdk.org/jfx/pull/2035/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2035&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8375243
  Stats: 21 lines in 1 file changed: 20 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jfx/pull/2035.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2035/head:pull/2035

PR: https://git.openjdk.org/jfx/pull/2035