Improve documentation for chutney warnings in "make test-network-all"

It appears that in fb32c522, we added a second call to test-network.sh inside our test-network-all loop. Now the code looks like this:

	for f in $$flavors; do \
		$(SHELL) $(top_srcdir)/test-driver --test-name $$f --log-file $(TEST_NETWORK_ALL_LOG_DIR)/$$f.log --trs-file $(TEST_NETWORK_ALL_LOG_DIR)/$$f.trs $(TEST_NETWORK_ALL_DRIVER_FLAGS) $(top_srcdir)/src/test/test-network.sh --flavor $$f $(TEST_NETWORK_FLAGS); \
		$(top_srcdir)/src/test/test-network.sh $(TEST_NETWORK_WARNING_FLAGS); \
	done; \

I might be wrong, but it looks to me like we're calling test-network.sh twice in each loop: once through test-driver, and once directly.

I'm not going to work on this till teor is back, though, since there are dragons here that I do not understand.