RFR: JDK-8285869: Selective cleanup in doclint Checker class

[Jonathan Gibbons]

Please review some localized cleanup for the doclint Checker class, primarily focused on upgrading to the use of "enhanced `switch`"

The output of one test was changed because of some improvements in one switch statement to eliminate the use of fall-through semantics.

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Commit messages:
 - JDK-8285869: Selective cleanup in doclint Checker class

Changes: https://git.openjdk.java.net/jdk/pull/8460/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8460&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285869
  Stats: 146 lines in 2 files changed: 12 ins; 39 del; 95 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8460.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8460/head:pull/8460

PR: https://git.openjdk.java.net/jdk/pull/8460