Add JavaDoc to Profiler API

spark-api/src/main/java/me/lucko/spark/api/profiler/Profiler.java

@@ -1,8 +1,23 @@
 package me.lucko.spark.api.profiler;
 
+import org.jetbrains.annotations.Nullable;
+
 public interface Profiler {
 
+    /**
+     * Start the Spark Profiler using a profiler configuration.
+     * A configuration can be built using {@link ProfilerConfiguration#builder()}.
+     *
+     * @param configuration the configuration object
+     * @return if the profiler started successfully
+     */
     boolean start(ProfilerConfiguration configuration);
 
+    /**
+     * Stop the currently running profiler.
+     *
+     * @return the profiler report or null if no profiler was running.
+     */
+    @Nullable
     ProfilerReport stop();
 }