RFR: JDK-8296287: Improve documentation for Types.directSupertypes()

[Joe Darcy]

Please review the PR and CSR (https://bugs.openjdk.org/browse/JDK-8296354) to add text to explicitly specify the behavior of Types.directSupertypes on java.lang.Object.

(This text could be added as informative text as it is implied by the existing specification, but I think it was reasonable to add the text as a normative statement instead.)

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

Commit messages:
 - JDK-8296287: Improve documentation for Types.directSupertypes()

Changes: https://git.openjdk.org/jdk/pull/10977/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10977&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296287
  Stats: 59 lines in 2 files changed: 59 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/10977.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10977/head:pull/10977

PR: https://git.openjdk.org/jdk/pull/10977