Show list of transition features on dlang.org and the man page

## Seb reported this on 2018-03-19T23:18:55Z

### Transferred from https://issues.dlang.org/show_bug.cgi?id=18635

### Description

````markdown
This file generates the compiler switches: https://github.com/dlang/dlang.org/blob/master/ddoc/source/preprocessor.d
````