Add javadocs for RunnerConfig class

core/src/main/java/feast/core/job/option/RunnerConfig.java

@@ -22,7 +22,23 @@
 import java.util.List;
 import java.util.Map;
 
+/**
+ * Value class containing the application-default configuration for a runner.
+ * When a job is started by core, all fields in the object will be converted into
+ * --key=value args to seed the beam pipeline options.
+ *
+ */
 public abstract class RunnerConfig {
+
+  /**
+   * Converts the fields in this class to a list of --key=value args to be
+   * passed to a {@link org.apache.beam.sdk.options.PipelineOptionsFactory}.
+   *
+   * Ignores values that are proto-default (e.g. empty string, 0).
+   *
+   * @return Array of string args in the format --key=value.
+   * @throws IllegalAccessException
+   */
   public String[] toArgs() throws IllegalAccessException {
     List<String> args = new ArrayList<>();
     for (Field field : this.getClass().getFields()) {