Since the function "doesn't exist", we can't link to it. Arguably, the
docs should only be tested with all features enabled, but for now just
don't try to link to `native_tls`.

rust-lang/rust#72081

The standard syntax doesn't work:
rust-lang/rust#86120

With the extra "helpful" bound, we also end up enforcing that C
implements Service, but since we're in a builder, C may not have been
set yet, and may be (), which in turn means that it isn't Service. So
users would end up with an error if they write:

    Builder::new().middleware_fn(|r| r).https().build()

but it would work with

    Builder::new().https().middleware_fn(|r| r).build()

which is silly.