update specs

Author: jvdp1

doc/specs/stdlib_string_type.md

@@ -1537,7 +1537,8 @@ Pure subroutine (Elemental subroutine, only when both `from` and `to` are `type(
 - `from`: Character scalar or [[stdlib_string_type(module):string_type(type)]].
   This argument is `intent(inout)`.
 - `to`: Character scalar or [[stdlib_string_type(module):string_type(type)]].
-  This argument is `intent(out)`.
+  This argument is `intent(inout)` when both `from` and `to` are `type(string_type)`,
+  otherwise `intent(out)`.
 
 #### Example