Merge pull request #6778 from oliver-sanders/psutil-docs++

psutil: document kwarg limitation

Author: wxtim

cylc/flow/cfgspec/globalcfg.py

@@ -893,7 +893,10 @@ def default_for(
 
                 This should be a multiline string containing Python expressions
                 to rank and/or filter hosts. All `psutil`_ attributes are
-                available for use in these expressions.
+                available for use in these expressions, e.g, ``cpu_percent``.
+                You can supply these with positional arguments e.g,
+                ``cpu_percent(1)``, however, keyword arguments will not work
+                e.g, ``cpu_percent(interval=1)``.
 
                 .. rubric:: Ranking