RFR: JDK-8268420: new Reporter method to report a diagnostic within a DocTree node [v6]

[Jonathan Gibbons]

> Please review an update to add a new method in Reporter to report a diagnostic within a DocTree node for those DocTree nodes that wrap a string.
> 
> This is the last of the current round of updates to improve the diagnostics that can be generated by javadoc.
> 
> The general fix, in JavadocLog and Reporter, is pretty simple, given all the improvements in recent related changes.
> 
> There are some cosmetic cleanups that were made while exploring the current solution.
> 
> The test is "reasonably thorough" and uses a custom taglet to generate diagnostics for selected nodes in doc comment trees. The test then "algorithmically validates" (i.e. no golden files or text blocks) the diagnostics that are either passed to a DiagnosticListener or written to the console stream.

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

  fix typos

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4489/files
  - new: https://git.openjdk.java.net/jdk/pull/4489/files/d124eb2b..435ae6a2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4489&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4489&range=04-05

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4489.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4489/head:pull/4489

PR: https://git.openjdk.java.net/jdk/pull/4489