checker: improve the nil fn error a bit

vlib/v/checker/struct.v

@@ -77,7 +77,7 @@ fn (mut c Checker) struct_decl(mut node ast.StructDecl) {
 				if sym.kind == .function {
 					if !field.typ.has_flag(.option) && !field.has_default_expr
 						&& field.attrs.filter(it.name == 'required').len == 0 {
-						error_msg := 'uninitialized `fn` struct fields are not allowed, since they can result in segfaults; use `?fn` or initialize the field with `=` (if you absolutely want to have unsafe function pointers, use `= unsafe { nil }`)'
+						error_msg := 'uninitialized `fn` struct fields are not allowed, since they can result in segfaults; use `?fn` or `[required]` or initialize the field with `=` (if you absolutely want to have unsafe function pointers, use `= unsafe { nil }`)'
 						c.note(error_msg, field.pos)
 					}
 				}

vlib/v/checker/tests/fn_check_for_matching_option_result_in_fields.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/fn_check_for_matching_option_result_in_fields.vv:2:2: notice: uninitialized `fn` struct fields are not allowed, since they can result in segfaults; use `?fn` or initialize the field with `=` (if you absolutely want to have unsafe function pointers, use `= unsafe { nil }`)
+vlib/v/checker/tests/fn_check_for_matching_option_result_in_fields.vv:2:2: notice: uninitialized `fn` struct fields are not allowed, since they can result in segfaults; use `?fn` or `[required]` or initialize the field with `=` (if you absolutely want to have unsafe function pointers, use `= unsafe { nil }`)
     1 | struct Abc {
     2 |     f fn (voidptr)
       |     ~~~~~~~~~~~~~~

vlib/v/checker/tests/generics_struct_init_err.out

@@ -1,4 +1,4 @@
-vlib/v/checker/tests/generics_struct_init_err.vv:14:2: notice: uninitialized `fn` struct fields are not allowed, since they can result in segfaults; use `?fn` or initialize the field with `=` (if you absolutely want to have unsafe function pointers, use `= unsafe { nil }`)
+vlib/v/checker/tests/generics_struct_init_err.vv:14:2: notice: uninitialized `fn` struct fields are not allowed, since they can result in segfaults; use `?fn` or `[required]` or initialize the field with `=` (if you absolutely want to have unsafe function pointers, use `= unsafe { nil }`)
    12 | 
    13 | struct FnHolder2[T] {
    14 |     func fn (int) int