MAINT add docstring to explain the motivation of the fixture

doc/tutorial/text_analytics/working_with_text_data_fixture.py

@@ -1,3 +1,13 @@
+"""Fixture module to skip the datasets loading when offline
+
+The 20 newsgroups data is rather large and some CI workers such as travis are
+stateless hence will not cache the dataset as regular sklearn users would do.
+
+The following will skip the execution of the working_with_text_data.rst doctests
+is the proper environment variable is configured (see the source code of
+check_skip_network for more details).
+
+"""
 from sklearn.utils.testing import check_skip_network