[FLINK-10596][cep, docs] Added description of TimeContext to docs

docs/dev/libs/cep.md

@@ -311,6 +311,8 @@ middle.oneOrMore()
 previously accepted events for a given potential match. The cost of this operation can vary, so when implementing
 your condition, try to minimize its use.
 
+Described context gives one access to event time characteristics as well. For more info see [Time context](#time-context).
+
 **Simple Conditions:** This type of condition extends the aforementioned `IterativeCondition` class and decides
 whether to accept an event or not, based *only* on properties of the event itself.
 
@@ -1508,6 +1510,7 @@ class MyPatternProcessFunction<IN, OUT> extends PatternProcessFunction<IN, OUT>
 
 The `PatternProcessFunction` gives access to a `Context` object. Thanks to it, one can access time related
 characteristics such as `currentProcessingTime` or `timestamp` of current match (which is the timestamp of the last element assigned to the match).
+For more info see [Time context](#time-context).
 Through this context one can also emit results to a [side-output]({{ site.baseurl }}/dev/stream/side_output.html).
 
 
@@ -1594,7 +1597,9 @@ val timeoutResult: DataStream<TimeoutEvent> = result.getSideOutput(outputTag)
 </div>
 </div>
 
-## Handling Lateness in Event Time
+## Time in CEP library
+
+### Handling Lateness in Event Time
 
 In `CEP` the order in which elements are processed matters. To guarantee that elements are processed in the correct order when working in event time, an incoming element is initially put in a buffer where elements are *sorted in ascending order based on their timestamp*, and when a watermark arrives, all the elements in this buffer with timestamps smaller than that of the watermark are processed. This implies that elements between watermarks are processed in event-time order.
 
@@ -1620,7 +1625,6 @@ SingleOutputStreamOperator<ComplexEvent> result = patternStream
 
 DataStream<String> lateData = result.getSideOutput(lateDataOutputTag);
 
-
 {% endhighlight %}
 
 </div>
@@ -1646,6 +1650,39 @@ val lateData: DataStream<String> = result.getSideOutput(lateDataOutputTag)
 </div>
 </div>
 
+### Time context
+
+In [PatternProcessFunction](#selecting-from-patterns) as well as in [IterativeCondition](#conditions) user has access to a context
+that implements `TimeContext` as follows:
+
+{% highlight java %}
+/**
+ * Enables access to time related characteristics such as current processing time or timestamp of
+ * currently processed element. Used in {@link PatternProcessFunction} and
+ * {@link org.apache.flink.cep.pattern.conditions.IterativeCondition}
+ */
+@PublicEvolving
+public interface TimeContext {
+
+ /**
+  * Timestamp of the element currently being processed.
+  *
+  * <p>In case of {@link org.apache.flink.streaming.api.TimeCharacteristic#ProcessingTime} this
+  * will be set to the time when event entered the cep operator.
+  */
+ long timestamp();
+
+ /** Returns the current processing time. */
+ long currentProcessingTime();
+}
+{% endhighlight %}
+
+This context gives user access to time characteristics of processed events (incoming records in case of `IterativeCondition` and matches in case of `PatternProcessFunction`).
+Call to `TimeContext#currentProcessingTime` always gives you the value of current processing time and this call should be preferred to e.g. calling `System.currentTimeMillis()`.
+
+In case of `TimeContext#timestamp()` the returned value is equal to assigned timestamp in case of `EventTime`. In `ProcessingTime` this will equal to the point of time when said event entered
+cep operator (or when the match was generated in case of `PatternProcessFunction`). This means that the value will be consistent across multiple calls to that method.
+
 ## Examples
 
 The following example detects the pattern `start, middle(name = "error") -> end(name = "critical")` on a keyed data