Error Prone: Enable checks for core plugins

Also document how to enable the checks for custom added plugins.

Change-Id: Ifde569cfd07820de0beaf213e0b69777063b7c64

Documentation/dev-build-plugins.txt

@@ -75,6 +75,12 @@ bazel-genfiles/plugins/<plugin-name>/<plugin-name>.jar
 Some plugins describe their build process in `src/main/resources/Documentation/build.md`
 file. It may worth checking.
 
+=== Error Prone checks
+
+Error Prone checks are enabled by default for core Gerrit and all core plugins. To
+enable the checks for custom plugins, add it in the `error_prone_packages` group
+in `tools/BUILD`.
+
 === Plugins with external dependencies ===
 
 If the plugin has external dependencies, then they must be included from Gerrit's

tools/BUILD

@@ -100,5 +100,16 @@ package_group(
     packages = [
         "//java/...",
         "//javatests/...",
+        "//plugins/codemirror-editor/...",
+        "//plugins/commit-message-length-validator/...",
+        "//plugins/delete-project/...",
+        "//plugins/download-commands/...",
+        "//plugins/gitiles/...",
+        "//plugins/hooks/...",
+        "//plugins/plugin-manager/...",
+        "//plugins/replication/...",
+        "//plugins/reviewnotes/...",
+        "//plugins/singleusergroup/...",
+        "//plugins/webhooks/...",
     ],
 )