Document the usage scenario of "as" keyword for trait coercion

[cher-nov]

I've just stumbled about such an error:

an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object

It seems that the second part of it is about syntax like this:

<SomeType as Into<i32>>::into( ... ))

But these scenarios don't seem to be documented on the as page in the official docs:
https://doc.rust-lang.org/std/keyword.as.html

It states that as can be used only to "cast between types, or rename an import".

[memoryruins]

The second part is about how as expressions can do coercions to trait objects. The reference page linked at the end of the rustdoc page mentions this, but it might help to mention it on the rustdoc page too.

For <SomeType as Into<i32>>, as is part of the syntax of qualified paths rather being about casts or renaming imports. A note could be added to the rustdoc page about this additional usage.

@rustbot label +A-docs +C-enhancement