RFR: JDK-8254052: improve type specificity of TagletWriter and friends [v5]

[Jonathan Gibbons]

> The types of `DocTree` parameters in some methods in `TagletWriter` can be changed from `DocTree` to more specific
> subtypes. Doing so makes the code less prone to incorrect calls, and removes the need for downcasting the argument in
> the method bodies.  This means the implementation of those methods can often be simplified.
> 
> In addition, the call sites need to be improved, and with the help of some improvements to methods in `Utils`, the
> methods that access different kinds of trees can return the lists using a more specific subtype as well.
> Finally, a quick analysis of some of the `Utils` methods to access different kinds of trees shows that there are some
> more convenient methods that can be used instead.

Jonathan Gibbons has updated the pull request incrementally with one additional commit since the last revision:

  post-merge fix

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/517/files
  - new: https://git.openjdk.java.net/jdk/pull/517/files/46263d60..b6c1ecae

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=517&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=517&range=03-04

  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/517.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/517/head:pull/517

PR: https://git.openjdk.java.net/jdk/pull/517