Improve language in 'Constant Expressions'.

Author: chriskrycho

src/const_eval.md

@@ -6,13 +6,13 @@ can be evaluated at compile-time.
 
 ## Constant expressions
 
-Certain types of expressions can be evaluated at compile time. These are called
-_constant expressions_ and are required in const contexts. In
-other places, such as in [`let` statements](statements.html#let-statements),
-constant expressions may be evaluated at compile time. If errors, such as out
-of bounds [array indexing] or [overflow] occurs,
-then it is a compiler error if the value must be evaluated at compile time,
-otherwise it is just a warning, but the code will most likely panic when run.
+Certain types of expressions, called constant expressions, can be evaluated at
+compile time. In [const contexts](#const-context), these are the only allowed
+expressions, and are always evaluated at compile time. In other places, such as
+in [`let` statements](statements.html#let-statements), constant expressions are
+Errors such as out of bounds [array indexing] or [overflow] are compiler errors
+if the value must be evaluated at compile time (i.e. in const contexts);
+otherwise they are just warnings, but the code will most likely panic when run.
 
 The following expressions are constant expressions, so long as any operands are
 also constant expressions and do not cause any [`Drop::drop`][destructors] calls