Add remarks regarding the initial content of span in string.Create (#4028)

* Add remarks regarding the initial content of span in string.Create

* Apply suggestions

Co-Authored-By: stephentoub <[email protected]>

* Apply suggestions

Co-Authored-By: Jeremy Barton <[email protected]>

Co-authored-by: stephentoub <[email protected]>
Co-authored-by: Jeremy Barton <[email protected]>

Author: 3 people

xml/System/String.xml

@@ -4493,7 +4493,12 @@ Depending on Why you want to call the `Copy` method, there are a number of alter
         <param name="action">A callback to initialize the string.</param>
         <summary>Creates a new string with a specific length and initializes it after creation by using the specified callback.</summary>
         <returns>The created string.</returns>
-        <remarks>To be added.</remarks>
+        <remarks>
+          <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.
+ ]]></format>
+        </remarks>
       </Docs>
     </Member>
     <Member MemberName="Empty">