Issue 8307 and 13336 - fix documentation around return type inference and ref deduction

[Issue 8307](https://issues.dlang.org/show_bug.cgi?id=8307) - inconsistent treatment of auto functions
[Issue 13336](https://issues.dlang.org/show_bug.cgi?id=13336) - auto ref return deduced to be ref despite return value coercion
This paragraph means that all use of "storage classes" (syntactically equivalent with "attributes") can make the function an auto-function.

```d
const foo() {}   // auto-function
pure nothrow foo() {}   // auto-function
```

If there's no storage class, you should use `auto` to declare auto-function.

```d
auto foo() {}
```