RFR: 8271090: Missing API docs in scenegraph classes

[Ajit Ghaisas]

This PR fixes javadoc warnings primarily in javafx.graphics module along with a remaining few in javafx.fxml, javafx.base and javafx.media modules.

Note :
- The javadoc needs to be generated with the JDK 18 EA build.
- There are still few remaining warnings in these modules. The root cause is different and they will be addressed under [JDK-8270996](https://bugs.openjdk.java.net/browse/JDK-8270996)

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Commit messages:
 - 8271090 - fix javadoc warnings - base,media,fxml
 - 8271090 - fix javadoc warnings

Changes: https://git.openjdk.java.net/jfx/pull/650/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=650&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271090
  Stats: 107 lines in 29 files changed: 74 ins; 0 del; 33 mod
  Patch: https://git.openjdk.java.net/jfx/pull/650.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/650/head:pull/650

PR: https://git.openjdk.java.net/jfx/pull/650