Rename Fast-Aggregations to Aggregations (#460)

* Rename Fast-Aggregations to Aggregations

This request came during the release review process. To make
the client documentation more inline with the other documentation,
we are renaming `Fast Aggregations` to just `Aggregations`.

Also, improved the documentation a little bit for the Aggregations
section.

* make the text more clear

Author: mdumandag

docs/features.rst

@@ -29,7 +29,7 @@ features:
 - Entry Processor
 - Built-in Predicates
 - Listener with Predicate
-- Fast Aggregations
+- Aggregations
 - Projections
 - Near Cache Support
 - Programmatic Configuration

docs/using_python_client_with_hazelcast_imdg.rst

@@ -2074,13 +2074,13 @@ using the ``next_page()`` method.
 .. Note:: ``PagingPredicate``, also known as Order & Limit, is not supported in
     Transactional Context.
 
-Fast-Aggregations
+Aggregations
 ~~~~~~~~~~~~~~~~~
 
-Fast-Aggregations feature provides some aggregate functions, such as ``sum``,
-``average``, ``max``, and ``min``, on top of Hazelcast ``Map`` entries. Their
-performance is high since they run in parallel for each partition and are 
-highly optimized for speed and low memory consumption.
+Aggregations allow computing a value of some function (e.g ``sum`` or ``max``)
+over the stored map entries. The computation is performed in a fully
+distributed manner, so no data other than the computed function value is
+transferred to the client, making the computation fast.
 
 The ``aggregator`` module provides a wide variety of built-in aggregators. The
 full list is presented below: