Revise const context rules

Let's break this out into two rules and reword.

Co-authored-by: Eric Huss <eric@huss.org>

Author: traviscross

src/const_eval.md

@@ -14,7 +14,10 @@ Certain forms of expressions, called constant expressions, can be evaluated at
 compile time.
 
 r[const-eval.const-expr.const-context]
-In [const contexts](#const-context), these are the only allowed expressions, and they are always evaluated at compile time.
+Expressions in a [const context] must be constant expressions.
+
+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.