Update sets_fragment.txt

doc/sets_fragment.txt

@@ -1,6 +1,16 @@
 The set type models the mathematical notion of a set. The set's
-basetype can only be an ordinal type. The reason is that sets are implemented
-as high performance bit vectors.
+basetype can only be an ordinal type of a certain size, namely:
+  * ``int8``-``int16``
+  * ``uint8``/``byte``-``uint16``
+  * ``char``
+  * ``enum``
+or equivalent. The reason is that sets are implemented as high 
+performance bit vectors. Attempting to declare a set with a larger type will
+result in an error:
+
+.. code-block:: nim
+
+  var s: set[int64] # Error: set is too large
 
 Sets can be constructed via the set constructor: ``{}`` is the empty set. The
 empty set is type compatible with any concrete set type. The constructor