rustdoc should hide private members in a const definition

[nodakai]

• http://doc.rust-lang.org/nightly/std/time/constant.UNIX_EPOCH.html

pub const UNIX_EPOCH: SystemTime = SystemTime(time::UNIX_EPOCH)

Here, time::UNIX_EPOCH refers to a constant in libstd/sys/{unix,windows}/time.rs (depending on the compile config) and should not be exposed. Its type is platform-specific, and private, too.

Cf. the SystemTime type is displayed as

• http://doc.rust-lang.org/nightly/std/time/struct.SystemTime.html

pub struct SystemTime(_);

[ollie27]

Fixed by #53409. We don't show the values of consts anymore.