If you have a function with a return type listed as `auto`,
please thoroughly describe what interface the return value
provides.
"Returns a forward range of Elem", where Elem is a template
parameter, is fine, for instance.
But look at std.regex.RegexMatch.front:
"Functionality for processing subsequent matches of global
regexes via range interface"
It has an example that mentions a `hit` property. Other parts of
the RegexMatch interface mention `pre` and `post` properties.
What are they? It is a mystery!
Turns out that the return type is a Captures struct, and that
could be made explicit with little effort. (Such as
https://github.com/dlang/phobos/pull/5963.)
This *is* redundant. Looking at the source code lets you find the
return type in less than thirty seconds. Or you can put into your
code: `pragma(msg, ReturnType!(func).stringof);` However, I
imagine we'd lose a lot of people in the "see missing
documentation" → "look at source code" step.
