RFR: 8274071: Clean up java.lang.ref comments and documentation

[Pavel Rappo]

This PR fixes an inline comment typo and reduces "overlinking" in a doc comment in `java.lang.ref.Reference`. Overlinking happens because the `reachabilityFence` method:
 * Links `package-summary.html#reachability` twice.
 * Refers to JLS 12.6 twice: once from a block tag and once from an inline tag.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/5609/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5609&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274071
  Stats: 4 lines in 1 file changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5609.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5609/head:pull/5609

PR: https://git.openjdk.java.net/jdk/pull/5609