Clarify that 'tests' data are baseline images

Author: ArchangeGabriel

setup.cfg.template

@@ -12,8 +12,8 @@
 
 [packages]
 # There are a number of data subpackages from Matplotlib that are
-# considered optional. All except tests data are installed by default,
-# but that can be changed here.
+# considered optional. All except 'tests' data (meaning the baseline
+# image files) are installed by default, but that can be changed here.
 #tests = False
 #sample_data = True