docs: Inaccessible/private types are shown under `Show` implementors

[japaric]

Head to Show doc page. You'll see the following implementors:

• Flavor (sync::mutex)
• Failure (sync::comm::sync)

The former is a private type and the latter is inaccessible (comm::sync is private). These two shouldn't show in the docs.

[alexcrichton]

This may be fixed, but needs more investigation. I believe this is a dupe of #14586, however.