Clarify the unitialized nature of stackalloc memory. (#41630)

* Clarify the unitialized nature of stackalloc memory.

Responding to anonymous feedback.

* Update docs/csharp/language-reference/operators/stackalloc.md

* Apply suggestions from code review

* Apply suggestions from code review

* Apply suggestions from code review

Co-authored-by: Andy (Steve) De George <[email protected]>

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Co-authored-by: Andy (Steve) De George <[email protected]>

Author: BillWagner

docs/csharp/language-reference/operators/stackalloc.md

@@ -49,7 +49,10 @@ The amount of memory available on the stack is limited. If you allocate too much
 
 - Avoid using `stackalloc` inside loops. Allocate the memory block outside a loop and reuse it inside the loop.
 
-The content of the newly allocated memory is undefined. You should initialize it before the use. For example, you can use the <xref:System.Span%601.Clear%2A?displayProperty=nameWithType> method that sets all the items to the default value of type `T`.
+The content of the newly allocated memory is undefined. You should initialize it, either with a `stackalloc` initializer, or a method like <xref:System.Span%601.Clear%2A?displayProperty=nameWithType> before it's used.
+
+> [!IMPORTANT]
+> Not initializing memory allocated by `stackalloc` is an important difference from the `new` operator. Memory allocated using the `new` operator is initialized to the 0 bit pattern.
 
 You can use array initializer syntax to define the content of the newly allocated memory. The following example demonstrates various ways to do that: