rustdoc: notable traits triggers on all &[T] and &mut [T] returns

[jsha]

All methods and functions that return &[T] get the ⓘ Notable Traits icon indicating the return type has some trait that the reader should know about immediately. For instance:

https://doc.rust-lang.org/nightly/std/vec/struct.Vec.html#method.as_slice

This shouldn't happen for two reasons:

• for generic T there's no reason to believe T will be u8, and implying to the user that it will be u8 is just confusing.
• even when T is u8, the Read and Write impls are far from the most notable thing about &[u8] and &mut [u8]. Rather than attaching a ⓘ to every instance of -> &[u8] and -> &mut [u8] in the documentation, we should rely on the slice page to communicate those impls.

Related: #100320, #55082