Clarify documentation about "except clauses"

They're called type conversions. Type casts are done with cast[T](...)
and are not type safe.

doc/manual/exceptions.txt

@@ -83,7 +83,7 @@ raised:
 
 Note that ``getCurrentException`` always returns a ``ref Exception``
 type. If a variable of the proper type is needed (in the example
-above, ``IOError``), one must use an explicit cast:
+above, ``IOError``), one must convert it explicitly:
 
 .. code-block:: nim
   try: