Improve docc intro

Author: Prachi Gauriar

Sources/DevTesting/Documentation.docc/Documentation.md

@@ -1,11 +1,23 @@
 # ``DevTesting``
 
-A small package package that contains useful additions for writing tests with Swift Testing. 
+Write repeatable tests with random data and create mock objects with powerful stubbing.  
 
 
 ## Overview
 
-DevTesting is a small testing package that makes it easy to test with random values and to stub and spy on mock objects.
+DevTesting is a small testing package that contains useful additions for writing tests with Swift Testing. Its 
+functionality is divided into two distinct subsystems: random value generation and stubbing. 
+
+The random value generation subsystem includes types and extensions that enable repeatable testing with random data. It
+includes ``SeedableRandomNumberGenerator``, which will always produce the same sequence of random numbers given the same
+seed. ``RandomValueGenerating`` uses a seedable random number generator to repeatably generate random values in your 
+automated tests. It captures the seed before every test so that you can re-run failed tests with the exact same random
+values. It also includes convenient functions for generating random booleans, numeric types, strings, optionals, UUIDs,
+data, URLs, and more.
+
+The stubbing subsystem includes ``Stub`` and ``ThrowingStub``, which make it easy to mock dependencies and verify their 
+behavior in tests. The system is designed to help you isolate the code under test while maintaining complete control 
+over how its dependencies behave and providing detailed information about how those dependencies were used.
 
 
 ## Topics