Add note about type-checking dynamic expressions in PGF2 Haddock

Closes #72

src/runtime/haskell-bind/PGF2.hsc

@@ -50,6 +50,8 @@ module PGF2 (-- * PGF
              mkType, unType,
 
              -- ** Type checking
+             -- | Dynamically-built expressions should always be type-checked before using in other functions,
+             -- as the exceptions thrown by using invalid expressions may not catchable.
              checkExpr, inferExpr, checkType,
 
              -- ** Computing