Revise wording on constraint-nonempty

Let's revise the wording to avoid a possible ambiguity about what
we're matching against.

Author: traviscross

src/patterns.md

@@ -555,7 +555,7 @@ It is written as `..=` followed by the upper bound.
 For example, `..=10` will match any integer less than or equal to 10, such as 10, 1, 0, and for signed integer types, all negative values.
 
 r[patterns.range.constraint-nonempty]
-A range pattern must match at least one possible value. In other words:
+A range pattern must be nonempty; it must span at least one value in the set of possible values for its type. In other words:
 
 * In `a..=b`, a ≤ b must be the case. For example, it is an error to have a range pattern `10..=0`, but `10..=10` is allowed.
 * In `a..b`, a < b must be the case. For example, it is an error to have a range pattern `10..0` or `10..10`.