Clarified when foreach can be used with ref

[pkulikov]

This PR is the result of this comment

I've disabled interactivity of two snippets, because the one with a span reports the following error:

(2,24): error CS1579: foreach statement cannot operate on variables of type 'Span' because 'Span' does not contain a public definition for 'GetEnumerator'

And the one with ref modifiers reports:

(1,21): error CS1525: Invalid expression term 'stackalloc'
(3,26): error CS1579: foreach statement cannot operate on variables of type 'Span' because 'Span' does not contain a public definition for 'GetEnumerator'
(3,10): error CS1073: Unexpected token 'ref'
(4,5): error CS1656: Cannot assign to 'item' because it is a 'foreach iteration variable'
(6,35): error CS1579: foreach statement cannot operate on variables of type 'Span' because 'Span' does not contain a public definition for 'GetEnumerator'
(6,10): error CS1073: Unexpected token 'ref'
(6,23): error CS0825: The contextual keyword 'var' may only appear within a local variable declaration or in script code

Use rich diff for review.

[BillWagner]

Thanks for spotting and submitting the PR @pkulikov

We appreciate it. I've reviewed the changes, and I'll now.