RFR: 8273998: Clarify specification for Window properties controlled by the window manager

[Kevin Rushforth]

Update the API specification for the `Window` and `Stage` classes to clarify that the values for some properties and methods can be changed or ignored by the platform. Several of the properties already have a comment to the effect that the value can change externally, and thus the properties are not bindable, but this should be clarified further and for all such properties and methods.

See the Description of the JBS issues for this bug and its associated CSR for more details.

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Commit messages:
 - additional doc updates
 - 8273998: Clarify specification for Window properties controlled by the window manager

Changes: https://git.openjdk.java.net/jfx/pull/708/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=708&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273998
  Stats: 84 lines in 2 files changed: 59 ins; 4 del; 21 mod
  Patch: https://git.openjdk.java.net/jfx/pull/708.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/708/head:pull/708

PR: https://git.openjdk.java.net/jfx/pull/708