Add a better explanation for the network marker

Co-authored-by: Florimond Manca <15911462+florimondmanca@users.noreply.github.com>

setup.cfg

@@ -18,7 +18,7 @@ combine_as_imports = True
 addopts = -rxXs
 markers =
   copied_from(source, changes=None): mark test as copied from somewhere else, along with a description of changes made to accodomate e.g. our test setup
-  network: marks tests which require network connection
+  network: marks tests which require network connection. Used in 3rd-party build environments that have network disabled.
 
 [coverage:run]
 omit = venv/*