Fine tune Spack tutorial text

Author: Vasileios Karakasis

docs/tutorial_build_automation.rst

@@ -109,14 +109,13 @@ Here is the test's code:
 
 
 When :attr:`~reframe.core.pipeline.RegressionTest.build_system` is set to ``'Spack'``, ReFrame will leverage Spack environments in order to build the test code.
-If the environment is not specified by the user, ReFrame will automatically create one inside the stage directory.
-That's the case in this example, where the software to be built is specified through the attribute :attr:`~reframe.core.buildsystems.Spack.specs`
-
-When Spack environments are specified by the user, ReFrame treats them as *test resources* so it expects to find them under the test's :attr:`~reframe.core.pipeline.RegressionTest.sourcesdir`, which defaults to ``'src'``.
+By default, ReFrame will create a new Spack environment in the test's stage directory and add the requested :attr:`~reframe.core.buildsystems.Spack.specs` to it.
+Users may also specify an existing Spack environment by setting the :attr:`~reframe.core.buildsystems.Spack.environment` attribute.
+In this case, ReFrame treats the environment as a *test resource* so it expects to find it under the test's :attr:`~reframe.core.pipeline.RegressionTest.sourcesdir`, which defaults to ``'src'``.
 
 As with every other test, ReFrame will copy the test's resources to its stage directory before building it.
-ReFrame will then activate the generated environment, add the given specs using the ``spack add`` command  and install them.
-Here is what ReFrame generates as a build script in this example:
+ReFrame will then activate the generated environment (either the one provided by the user or the one generated by ReFrame), add the given specs using the ``spack add`` command  and, finally, install the packages in the environment.
+Here is what ReFrame generates as a build script for this example:
 
 .. code:: bash