make explicit the scope of config's gather_subset

it only affects the invocation of setup triggered by the gather_facts directive in plays (explicit or implicit)

examples/ansible.cfg

@@ -33,7 +33,8 @@
 # explicit - do not gather by default, must say gather_facts: True
 #gathering = implicit
 
-# by default retrieve all facts subsets
+# This only affects the gathering done by a play's gather_facts directive,
+# by default gathering retrieves all facts subsets
 # all - gather all subsets
 # network - gather min and network facts
 # hardware - gather hardware facts (longest facts to retrieve)