[Issue 2420] string mixins are not considered in ddoc

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=2420

--- Comment #6 from FeepingCreature <default_357-line at yahoo.de> ---
It's weird... if I look at Ddoc for a string mixin of aliases in a struct, I
see the alias symbols (by name), but not any comments the mixin puts on them.

--