RFR: 8282756: Make ElementKind checks more specific

[Pavel Rappo]

Out of all executable elements, inherit documentation only for methods.

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Commit messages:
 - Match methods instead of executable elements
 - Refactor Utils.isAnnotationType
 - Use methods from ElementKind

Changes: https://git.openjdk.java.net/jdk/pull/7747/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7747&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282756
  Stats: 76 lines in 19 files changed: 2 ins; 34 del; 40 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7747.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7747/head:pull/7747

PR: https://git.openjdk.java.net/jdk/pull/7747