Enable error-prone checks by default

Based on recent work by Dave Borowitz on the Gerrit project [1, 2].

The warnings/error configuration is unchanged, but now the checks are
enabled by default during the build rather than having to be manually
invoked.

[1] https://gerrit-review.googlesource.com/c/gerrit/+/225653
[2] https://gerrit-review.googlesource.com/c/gerrit/+/225714

Change-Id: I112146911d3067bb5283cdf7b188be73f8f8f1e8

tools/BUILD

diff --git a/tools/BUILD b/tools/BUILD
index 00f2cbe..9533a6d 100644
--- a/tools/BUILD
+++ b/tools/BUILD
@@ -14,8 +14,10 @@
     visibility = ["//visibility:public"],
 )
 
-# This EP warnings list borrowed from here:
+# Error Prone errors enabled by default; see ../.bazelrc for how this is
+# enabled. This warnings list is originally based on:
 # https://github.com/bazelbuild/BUILD_file_generator/blob/master/tools/bazel_defs/java.bzl
+# However, feel free to add any additional errors. Thus far they have all been pretty useful.
 java_package_configuration(
     name = "error_prone",
     javacopts = [