doc review: use the term libraries more often than APIs

APIs are the interfaces that our libraries provide. So where we actually
want to talk about the full set of value that we provide to test users,
let's use the term libraries. And use APIs when it makes more sense to
do so.
Signed-off-by: NCleber Rosa <crosa@redhat.com>

docs/source/Introduction.rst

@@ -15,7 +15,7 @@ Avocado is composed by:
   language of choice, or be written in Python and use the available libraries. In both
   cases, you get facilities such as automated log and system information collection.
 
-* APIs that help you write tests in a concise, yet expressive way.
+* Libraries that help you write tests in a concise, yet expressive and powerful way.
   The Test API is the whole set of modules, classes and functions available
   under the :mod:`avocado` main module, excluding the :mod:`avocado.core`
   module and their submodules, which is part of application's infrastructure.