rustdoc: Show generic parameters for item being documented

[camelid]

Currently, rustdoc does not display generic parameters for the item being documented:

This can be confusing because you will see generic parameters used that are never documented themselves. This is especially common with enums. As you can see in the image above, the variants section for std::option::Option shows the Some(T) variant — but where did the T come from?

It might be nice to show something like this instead (the difference is the <T> added after std::option::Option in the header):

[jyn514]

@camelid it's shown if you hit 'Show declaration'
. I think that should be expanded by default - did you change something in settings.html?

[camelid]

Ah, I forgot about that feature. I still think there might be value in showing the generic parameters outside of the declaration section. Maybe it would help if the declaration were syntax-highlighted...