[hotfix] [docs] Improve description of keyBy().

docs/dev/stream/operators/index.md

@@ -100,9 +100,9 @@ dataStream.filter(new FilterFunction<Integer>() {
         <tr>
           <td><strong>KeyBy</strong><br>DataStream &rarr; KeyedStream</td>
           <td>
-            <p>Logically partitions a stream into disjoint partitions, each partition containing elements of the same key.
-            Internally, this is implemented with hash partitioning. See <a href="{{ site.baseurl }}/dev/api_concepts.html#specifying-keys">keys</a> on how to specify keys.
-            This transformation returns a KeyedStream.</p>
+            <p>Logically partitions a stream into disjoint partitions. All records with the same key are assigned to the same partition. Internally, <em>keyBy()</em> is implemented with hash partitioning. There are different ways to <a href="{{ site.baseurl }}/dev/api_concepts.html#specifying-keys">specify keys</a>.</p>
+            <p>
+            This transformation returns a <em>KeyedStream</em>, which is, among other things, required to use <a href="{{ site.baseurl }}/dev/stream/state/state.html#keyed-state">keyed state</a>. </p>
     {% highlight java %}
 dataStream.keyBy("someKey") // Key by field "someKey"
 dataStream.keyBy(0) // Key by the first element of a Tuple