Specify that range patterns must be nonempty.

This changes the reference to match the behavior of the compiler, based on my empirical testing.

See also rust-lang/rust#149165

Author: theemathas

src/patterns.md

@@ -554,10 +554,12 @@ 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-less-than]
-The lower bound cannot be greater than the upper bound.
-That is, in `a..=b`, a ≤ b must be the case.
-For example, it is an error to have a range pattern `10..=0`.
+r[patterns.range.constraint-nonempty]
+A range pattern must match at least one possible value. 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`.
+* In `..b`, b must not be the smallest value of its type. For example, it is an error to have a range pattern `..-128i8` or `..f64::NEG_INFINITY`.
 
 r[patterns.range.bound]
 A bound is written as one of: