RFR: 8311290: Improve java.lang.ref.Cleaner rendered documentation

[Pavel Rappo]

1. Missing leading asterisk interferes with example indentation; nothing in https://bugs.openjdk.org/browse/JDK-8276700 (or review thereof), which introduced that line, suggests that the asterisk was omitted on purpose. 
2. link and linkplain tags are used suboptimally.

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Commit messages:
 - Improve links markup

Changes: https://git.openjdk.org/jdk/pull/14768/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14768&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311290
  Stats: 9 lines in 1 file changed: 1 ins; 0 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/14768.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14768/head:pull/14768

PR: https://git.openjdk.org/jdk/pull/14768