CUG: doc param logical quirk

doc/src/cylc-user-guide/cug.tex

@@ -4779,6 +4779,29 @@ \subsubsection{Parameter Offsets In The Graph}
     graph = "proc_small => proc_big => proc_huge"
 \end{lstlisting}
 
+However, a quirk in the current system means that you should avoid mixing
+conditional logic in these statements. For example, the following will do the
+unexpected:
+
+\begin{lstlisting}
+    graph = foo<m-1> & baz => foo<m>  # for m = cat, dog
+\end{lstlisting}
+currently expands to:
+\begin{lstlisting}
+    graph = foo_cat & baz => foo_dog
+# when users may expect it to be:
+#    graph = foo_cat => foo_dog
+#    graph = baz => foo_cat & foo_dog
+\end{lstlisting}
+
+For the time being, writing out the logic explicitly will give you the correct
+graph.
+
+\begin{lstlisting}
+    graph = """foo<m-1> => foo<m>  # for m = cat, dog
+               baz => foo<m>"""
+\end{lstlisting}
+
 \subsubsection{Task Families And Parameterization}
 
 Task family members can be generated by parameter expansion: