Revise following sentence about eval outside of const

The changes to the sentence before this one make the lack of clarity
of this sentence stand out more, so let's improve the clarity of this
sentence about evaluation of constant expressions outside of a const
context.

Author: traviscross

src/const_eval.md

@@ -20,7 +20,7 @@ r[const-eval.const-expr.evaluation]
 Expressions in const contexts are always evaluated at compile time.
 
 r[const-eval.const-expr.runtime-context]
-In other places, such as [let statements], constant expressions *may* be, but are not guaranteed to be, evaluated at compile time.
+Outside of const contexts, constant expressions *may* be, but are not guaranteed to be, evaluated at compile time.
 
 r[const-eval.const-expr.error]
 Behaviors such as out of bounds [array indexing] or [overflow] are compiler errors if the value