RFR: JDK-8283594: Improve docs of ElementScanner classes

[Joe Darcy]

Simple update to add links. In the subclasses, the erasure of the parameter's type needs to be used for the link to be recognized as valid.

After getting reviews, I'll reflow the paragraphs before pushing.

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Commit messages:
 - JDK-8283594: Improve docs of ElementScanner classes

Changes: https://git.openjdk.java.net/jdk/pull/7932/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7932&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283594
  Stats: 9 lines in 5 files changed: 0 ins; 0 del; 9 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7932.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7932/head:pull/7932

PR: https://git.openjdk.java.net/jdk/pull/7932