RFR: JDK-8269774: doclint reports missing javadoc comments for JavaFX properties if the docs are on the property method

[Jonathan Gibbons]

Please review a simple change to the code for generating docs for JavaFX properties, in order to suppress an inappropriate
"missing comment" warning.

The change is to use `hasDocCommentTree` instead of `getDocCommentTree`.

A new test is added, that runs javadoc on similar classes, with and without a comment on the field for a property. In both cases, no warnings about missing comments should be generated, and when the comment is available, it is used as expected.

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

Commit messages:
 - fix whitespace
 - JDK-8269774: doclint reports missing javadoc comments for JavaFX properties if the docs are on the property method

Changes: https://git.openjdk.java.net/jdk/pull/4747/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4747&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269774
  Stats: 160 lines in 2 files changed: 159 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4747.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4747/head:pull/4747

PR: https://git.openjdk.java.net/jdk/pull/4747