RFR: JDK-8283594: Improve docs of ElementScanner classes

[Jonathan Gibbons]

On Wed, 23 Mar 2022 23:27:05 GMT, Joe Darcy <darcy at openjdk.org> wrote:

> 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.

In the implementation, it looks like the `visitXYZ` methods call the `scan(Iterable<? extends Element>, P p)` methods.

However, it is notable that the `Iterable` overload is `final`, (along with the 3rd overload that defaults the `P` parameter), thus effectively guaranteeing that the `scan` method that takes a single `Element` will indeed be called.

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Marked as reviewed by jjg (Reviewer).

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