RFR: 8252546: Move ObservableValue's equality check and lazy evaluation descriptions to @implSpec

[Nir Lisker]

Moving implementation details about lazy evaluation and equality checking to `@implSpec`.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jfx/pull/292/files
 Webrev: https://webrevs.openjdk.java.net/jfx/292/00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8252546
  Stats: 12 lines in 1 file changed: 6 ins; 3 del; 3 mod
  Patch: https://git.openjdk.java.net/jfx/pull/292.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/292/head:pull/292

PR: https://git.openjdk.java.net/jfx/pull/292