RFR: JDK-8280534: Enable compile-time doclint reference checking

[Joe Darcy]

The changes in this PR on top of the out-for-review changes in https://git.openjdk.java.net/jdk/pull/7222 allow compile-time doclint checking to be enabled in all JDK modules.

Typically, a @SuppressWarnings("doclint:refernce") annotation is added to declaration with javadoc blocks that have already had distinguished cross-module links added (JDK-8280492).

One exception is in src/java.base/share/classes/java/net/package-info.java where the cross-module link was (for now) removed. Currently the SuppressWarnings annotation type is not declared to allow its annotations to be applied to package declarations. I'll look into amending that, but in the mean time, I think it is beneficial for the JDK build, and the base module in particular, to have compile-time doclint protections turned on.

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

Commit messages:
 - Cover java.base.
 - JDK-8280534: Enable compile-time doclint reference checking

Changes: https://git.openjdk.java.net/jdk/pull/7237/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7237&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280534
  Stats: 28 lines in 21 files changed: 20 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7237.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7237/head:pull/7237

PR: https://git.openjdk.java.net/jdk/pull/7237