RFR: JDK-8267204: Expose access to underlying streams in Reporter

[Jonathan Gibbons]

Please review an update to `jdk.javadoc/jdk.javadoc.doclets.Reporter` to add 3 new methods and to improve the descriptions of other parts of the interface.

The new methods provide access to the underlying streams (informally, for standard output and diagnostic output), and a new `report` method to report diagnostics at an arbitrary position in a file being read by a doclet, or a taglet within a doclet.

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Commit messages:
 - Fix typos
 - Fix typos
 - Fix typos
 - Merge remote-tracking branch 'upstream/master' into jdk-8267204-reporter
 - Address review feedback
 - Address review feedback
 - extend Messages to provide convenient access to the new Reporter.report method
 - add new test; update stream handling
 - Merge remote-tracking branch 'upstream/master' into jdk-8267204-reporter
 - Merge remote-tracking branch 'upstream/master' into jdk-8267204-reporter
 - ... and 4 more: https://git.openjdk.java.net/jdk/compare/1ae934e0...521ffe99

Changes: https://git.openjdk.java.net/jdk/pull/4216/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4216&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8267204
  Stats: 541 lines in 10 files changed: 427 ins; 35 del; 79 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4216.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4216/head:pull/4216

PR: https://git.openjdk.java.net/jdk/pull/4216