As per the discussion at dlang#5963, we
want the option to swap out the types for Regex, Capture, etc. This
is impossible if people are able to refer to Regex by name.
Furthermore, according to @wilzbach, aliases are not an option.
Therefore, this PR deprecates public uses of the Regex types to allow
us to switch out the concrete type later.

Since it's apparently better to document the entire interface of your
return types than to use static typing to direct people to the type's
own documentation, this ensures that that documentation is complete.