Restore add(Promise) and addAll(Promise...) methods to PromiseCombiner.

Motivation:

A testing goof in 7c630fe introduced a binary incompatibility when the old Promise-specific `add` and `addAll` methods in PromiseCombiner were generalized to accept `Futures`.

Modification:

- Restore (but mark as `@Deprecated`) old PromiseCombiner methods.
- Fixed a couple minor documentation typos because sure why not.

Result:

`PromiseCombiner` is binary-compatible with previous versions of Netty.

common/src/main/java/io/netty/util/concurrent/PromiseCombiner.java

@@ -48,9 +48,22 @@ public final class PromiseCombiner {
         }
     };
 
+    /**
+     * Adds a new promise to be combined. New promises may be added until an aggregate promise is added via the
+     * {@link PromiseCombiner#finish(Promise)} method.
+     *
+     * @param promise the promise to add to this promise combiner
+     *
+     * @deprecated Replaced by {@link PromiseCombiner#add(Future)}.
+     */
+    @Deprecated
+    public void add(Promise promise) {
+        add((Future) promise);
+    }
+
     /**
      * Adds a new future to be combined. New futures may be added until an aggregate promise is added via the
-     * {@link PromiseCombiner#finish(Promise)} method has been called.
+     * {@link PromiseCombiner#finish(Promise)} method.
      *
      * @param future the future to add to this promise combiner
      */
@@ -61,9 +74,22 @@ public final class PromiseCombiner {
         future.addListener(listener);
     }
 
+    /**
+     * Adds new promises to be combined. New promises may be added until an aggregate promise is added via the
+     * {@link PromiseCombiner#finish(Promise)} method.
+     *
+     * @param promises the promises to add to this promise combiner
+     *
+     * @deprecated Replaced by {@link PromiseCombiner#addAll(Future[])}
+     */
+    @Deprecated
+    public void addAll(Promise... promises) {
+        addAll((Future[]) promises);
+    }
+
     /**
      * Adds new futures to be combined. New futures may be added until an aggregate promise is added via the
-     * {@link PromiseCombiner#finish(Promise)} method has been called.
+     * {@link PromiseCombiner#finish(Promise)} method.
      *
      * @param futures the futures to add to this promise combiner
      */