TST: add comment that plotting tests are slow (#38171)

Author: ivanovmg

pandas/tests/plotting/common.py

@@ -1,3 +1,10 @@
+"""
+Module consolidating common testing functions for checking plotting.
+
+Currently all plotting tests are marked as slow via
+``pytestmark = pytest.mark.slow`` at the module level.
+"""
+
 import os
 from typing import TYPE_CHECKING, Sequence, Union
 import warnings