Added sub heading to ignite.metrics (#2448)

* Added sub heading to ignite.metrics

* Updated docs for metrics

* updated docs for ignite.metrics

* improved the explanation of metric computation

* made minor changes to the texts

* made a small change to the sentence construction

* framed a sentence in a better way

Co-authored-by: vfdev <vfdev.5@gmail.com>

Author: sayantan1410

docs/source/metrics.rst

@@ -4,8 +4,17 @@ ignite.metrics
 Metrics provide a way to compute various quantities of interest in an online
 fashion without having to store the entire output history of a model.
 
-In practice a user needs to attach the metric instance to an engine. The metric
-value is then computed using the output of the engine's ``process_function``:
+Attach Engine API
+------------------
+
+The metrics as stated above are computed in a online fashion, which means that the metric instance accumulates some internal counters on 
+each iteration and metric value is computed once the epoch is ended. Internal counters are reset after every epoch. In practice, this is done with the 
+help of three methods: :meth:`~ignite.metrics.metric.Metric.reset()`, :meth:`~ignite.metrics.metric.Metric.update()` and :meth:`~ignite.metrics.metric.Metric.compute()`.
+
+Therefore, a user needs to attach the metric instance to the engine so that the above three methods can be triggered on execution of certain :class:`~ignite.engine.events.Events`.
+The :meth:`~ignite.metrics.metric.Metric.reset()` method is triggered on ``EPOCH_STARTED`` event and it is responsible to reset the metric to its initial state. The :meth:`~ignite.metrics.metric.Metric.update()` method is triggered
+on ``ITERATION_COMPLETED`` event as it updates the state of the metric using the passed batch output. And :meth:`~ignite.metrics.metric.Metric.compute()` is triggered on ``EPOCH_COMPLETED``
+event. It computes the metric based on its accumulated states. The metric value is computed using the output of the engine's ``process_function``:
 
 .. code-block:: python