RFR: JDK-8283594: Improve docs of ElementScanner classes [v3]

[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.

Joe Darcy has updated the pull request incrementally with one additional commit since the last revision:

  Fix existing be/by typo.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7932/files
  - new: https://git.openjdk.java.net/jdk/pull/7932/files/d0b65388..f0d889aa

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7932&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7932&range=01-02

  Stats: 5 lines in 5 files changed: 0 ins; 0 del; 5 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