RFR: JDK-8245058: improve presentation of annotations for modules and packages

[Hannes Wallnöfer]

JDK-8245058: improve presentation of annotations for modules and packages

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Commit messages:
 - Improve HtmlStyle documentation
 - JDK-8245058 unify signature generating code
 - Refactor and unify signature generating code
 - Add proper signatures to module and package pages

Changes: https://git.openjdk.java.net/jdk/pull/966/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=966&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8245058
  Stats: 1539 lines in 53 files changed: 611 ins; 446 del; 482 mod
  Patch: https://git.openjdk.java.net/jdk/pull/966.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/966/head:pull/966

PR: https://git.openjdk.java.net/jdk/pull/966