Document a behavior of string.Create

Author: tannergooding

xml/System/String.xml

@@ -4364,6 +4364,8 @@ The following example demonstrates the <xref:System.String.CopyTo%2A> method.
           <format type="text/markdown"><![CDATA[
 ## Remarks
 The initial content of the destination span passed to `action` is undefined. Therefore, it is the delegate's responsibility to ensure that every element of the span is assigned. Otherwise, the resulting string could contain random characters.
+
+In order to support interop scenarios, the underlying buffer is guaranteed to be at least 1 greater than represented by the span parameter of the action callback. This additional index represents the null-terminator and if written that is the only value supported, writing any value other than the null-terminator will result in corrupting the string and is considered undefined behavior.
  ]]></format>
         </remarks>
       </Docs>