RFR: JDK-8296287: Improve documentation for Types.directSupertypes() [v2]

[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.)

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Respond to review feedback.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10977/files
  - new: https://git.openjdk.org/jdk/pull/10977/files/e216112e..26ac86b8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10977&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10977&range=00-01

  Stats: 8 lines in 2 files changed: 0 ins; 0 del; 8 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