Add null to tostring docs

website/docs/language/functions/tostring.html.md

@@ -14,7 +14,7 @@ Explicit type conversions are rarely necessary in Terraform because it will
 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) can be converted to string.
+Only the primitive types (string, number, and bool) and `null` can be converted to string.
 All other values will produce an error.
 
 ## Examples
@@ -26,6 +26,8 @@ hello
 1
 > tostring(true)
 true
+> tostring(null)
+null
 > tostring([])
 Error: Invalid function argument