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
"have common" -> "must either be both r-values or both l-values"?