Add comments to FDv2 data source interfaces.

Author: kinyoklion

lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/datasources/Initializer.java

@@ -46,6 +46,8 @@
 public interface Initializer extends Closeable {
     /**
      * Run the initializer to completion.
+     * <p>
+     * This method is intended to be called only single time for an instance.
      * @return The result of the initializer.
      */
     CompletableFuture<FDv2SourceResult> run();

lib/sdk/server/src/main/java/com/launchdarkly/sdk/server/datasources/Synchronizer.java

@@ -52,6 +52,8 @@ public interface Synchronizer extends Closeable {
      * <p>
      * This method is intended to be driven by a single thread, and for there to be a single outstanding call
      * at any given time.
+     * <p>
+     *  Once SHUTDOWN, TERMINAL_ERROR, or GOODBYE has been produced, then no further calls to next() should be made.
      * @return a future that will complete when the next result is available
      */
     CompletableFuture<FDv2SourceResult> next();