RFR: 8277847: Support toolGuide tag in class-level documentation

[Julia Boes]

This change adds support for the `@toolGuide` tag in class-level API documentation. 

(A use case is the jwebserver tool, where the com.sun.net.httpserver.SimpleFileServer class provides API points for extension and customization of the underlying server and its components. Linking to the tool's man page in the class-level documentation would be beneficial.)

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

Commit messages:
 - initial commit

Changes: https://git.openjdk.java.net/jdk/pull/6566/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6566&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277847
  Stats: 10 lines in 1 file changed: 7 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6566.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6566/head:pull/6566

PR: https://git.openjdk.java.net/jdk/pull/6566