Reorganize and document errorprone flags.

The bazel errorprone plugin currently only enables default error checkers
https://github.com/bazelbuild/bazel/blob/97975603e5ff2247e6bb352e3afd27fea38f108d/src/java_tools/buildjar/java/com/google/devtools/build/buildjar/javac/plugins/errorprone/ErrorPronePlugin.java#L52

These are errorprone checkers listed under ENABLED_ERRORS at
https://github.com/google/error-prone/blob/c6f24bc387989158d99af28e7ae86755e56c5f38/core/src/main/java/com/google/errorprone/scanner/BuiltInCheckerSuppliers.java#L273

This change enables nearly all available errorprone checkers to
increase code style consistency. This corresponds to using allChecks()
from errorprone
https://github.com/google/error-prone/blob/c6f24bc387989158d99af28e7ae86755e56c5f38/core/src/main/java/com/google/errorprone/scanner/BuiltInCheckerSuppliers.java#L260

Reorganized errorprone flags to correspond with groups of checkers
defined within the errorprone sourcecode.