kiji-poms: POM-24. Fix and explain checkstyle configuration in root pom.

issue: https://jira.kiji.org/browse/POM-24
review: https://review.kiji.org/r/1385
commit: https://github.com/kijiproject/kiji-poms/commit/59fe5e6e41485e11dfa9fe0ad3581c4cdc3d2642