[Issue 18124] std.regex.RegexMatch's front property is under-documented

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=18124

--- Comment #8 from Steven Schveighoffer <schveiguy at yahoo.com> ---
PR: https://github.com/dlang/phobos/pull/5976

I opted only to fix RegexMatch.front, since all the other auto returns
explicitly say what they return.

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