Multiple InterruptHandler links in doc page for usart

[YuhanLiin]

In the doc page for usart, there are two links to InterruptHandler. Both redirect to the same page. Moreover, when I hover over the "Source" links on that page, they all point to usart/buffered.rs instead of usart/mod.rs.

I think the reason for this is because in mod.rs, there's the line pub use buffered::*, which pulls in the version of InterruptHandler from buffered.rs. This is probably what's messing up the documentation generation.