RFR: JDK-8268352: Rename javadoc Messager class to JavadocLog [v2]

[Jonathan Gibbons]

> Please review a simple cleanup, to rename the javadoc Messager class to the more explicit, less-confusing name of JavadocLog, since it is the javadoc subtype of the javac Log class.
> 
> The new name is in line with other javadoc subtypes of javac classes.
> 
> The change only affects the javadoc.tool package; it does not affect any doclet packages, which use the `Reporter` interface, provided by `JavadocLog`.

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

  address review feedback

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4405/files
  - new: https://git.openjdk.java.net/jdk/pull/4405/files/20bb75b2..c7f81586

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4405&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4405&range=00-01

  Stats: 4 lines in 4 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4405.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4405/head:pull/4405

PR: https://git.openjdk.java.net/jdk/pull/4405