RFR: 8268642: Expand the javafx.beans.property.Property documentation

[Michael Strauß]

* Expand the `Property.bind` and `Property.bindBidirectional` documentation
* Change the name of the formal parameter of `Property.bind` to "source" (currently, it is inconsistently named "observable", "rawObservable" or "newObservable")

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Commit messages:
 - Expanded Property documentation

Changes: https://git.openjdk.java.net/jfx/pull/533/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=533&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268642
  Stats: 190 lines in 18 files changed: 36 ins; 17 del; 137 mod
  Patch: https://git.openjdk.java.net/jfx/pull/533.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/533/head:pull/533

PR: https://git.openjdk.java.net/jfx/pull/533