Clarify :16sweet a bit

S02-bits.pod

@@ -3775,11 +3775,12 @@ C<:%a>) never take an argument and don't care what the next character
 is.  They are considered complete.  These forms require an identifier
 to serve as the key.
 
-For identifiers that take a numeric argument, it is allowed to
-abbreviate, for example, C<:sweet(16)> to C<:16sweet>.  (This is
+For identifiers that take a non-negative integer argument, it is allowed
+to abbreviate, for example, C<:sweet(16)> to C<:16sweet>. (This is
 distinguishable from the :16<deadbeef> form, which never has an
-alphabetic character following the number.)  Only literal decimal
-numbers may be swapped this way.  Please note that this abbreviation allows:
+alphabetic character following the number.) Only literal non-negative
+integers numbers may be swapped this way. Please note that this
+abbreviation allows:
 
   s:2nd/foo/bar/  # or 3rd, 4th, 5th etc.