Macro reference: Add hint to clear variable afterwards

[ci skip]

data/core/macros/unit-utils.cfg

@@ -202,6 +202,7 @@
     # Example where this is used to flip all orcs to whatever side James is on:
     #! {STORE_UNIT_VAR (id=James) side side_of_James}
     #! {MODIFY_UNIT (race=orc) side $side_of_James}
+    #! {CLEAR_VARIABLE side_of_James}
     [store_unit]
         [filter]
             {FILTER}