[jfx15] RFR: 8228570: Add various documentation clarifications [v3]

[Nir Lisker]

> Adds clarifications to the documentation in various places. Some notes:
> 
> * Point 6 should probably be deferred until it is verified that the tutorials are correct enough, seeing as they were
>   updated to Java 8 only.
> * Point 8 has been deferred until all the animation bugs have been resolevd.
> * Point 5: I wrote new documentation about the `extractor` for the `observableArrayList(Callback<E, Observable[]>
>   extractor)` method. Later I found that `observableList(List<E> list, Callback<E, Observable[]> extractor)` already
>   talks about it (I updated it too). I'm not sure which of them we want to keep, or maybe merge them.
> * Point 1: I think that it's necessary to mention the internal implementation behavior even if it requires a caveat that
>   this is only the current behavior and may change in the future. What constitutes a "change" is extremely important and
>   there is no way for the user to know it. I've tripped on this hard when using ReactFX which uses object equality
>   instead, so when the JavaFX observables are wrapped by ReactFX observables, the behavior changes silently.
> I think that in the future we will want to let the user define what a change is (for example, by creating an
> overridable method with the current behavior as the default, or using object equality and letting the user override
> that, although that's more risky). Even a `HashMap` that uses object equality has the sister implementation
> `IndentityHashMap` to deal with this ambiguity.

Nir Lisker has updated the pull request incrementally with one additional commit since the last revision:

  Addressed parts of the review comments

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

Changes:
  - all: https://git.openjdk.java.net/jfx/pull/276/files
  - new: https://git.openjdk.java.net/jfx/pull/276/files/a44bfb18..6c0a3eb9

Webrevs:
 - full: https://webrevs.openjdk.java.net/jfx/276/webrev.02
 - incr: https://webrevs.openjdk.java.net/jfx/276/webrev.01-02

  Stats: 24 lines in 3 files changed: 7 ins; 7 del; 10 mod
  Patch: https://git.openjdk.java.net/jfx/pull/276.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/276/head:pull/276

PR: https://git.openjdk.java.net/jfx/pull/276