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
Ah right, that's how to explain something. First tell people about exceptional cases, then declare the normal usage as exception of the exception :).