Update tostring.mdx for null value conversion

Per https://github.com/hashicorp/terraform/issues/33603, updating the documentation to accurately reflect the current implementation.

website/docs/language/functions/tostring.mdx

@@ -12,7 +12,7 @@ convert types automatically where required. Use the explicit type conversion
 functions only to normalize types returned in module outputs.
 
 Only the primitive types (string, number, and bool) and `null` can be converted to string.
-All other values will produce an error.
+`tostring(null)` produces a `null` value of type `string`. All other values will produce an error. 
 
 ## Examples
 
@@ -24,7 +24,7 @@ hello
 > tostring(true)
 true
 > tostring(null)
-null
+tostring(null)
 > tostring([])
 Error: Invalid function argument