Newlines don't cause a paragraph break when mixing `#[doc]` and `///`

[camelid]

See the test added in #91199. This code:

#[doc = "Hello world!\n\n"]
/// Goodbye!
#[doc = "  Hello again!\n"]
pub struct S1;

renders as:

Hello world! Goodbye! Hello again!

rather than the expected:

Hello world!

Goodbye! Hello again!

Meanwhile, changing some #[doc]s to ///s and vice versa causes it to render as expected.

cc @GuillaumeGomez

[GuillaumeGomez]

This is the expected behaviour actually: it converts the /// into a #[doc = "..."] directly. So if you don't put empty doc comments between, it's as expected.

[camelid]

But why are the newlines in the #[doc = "..."] stripped?

[GuillaumeGomez]

Oh, I didn't see that. I think it's because we use trim to remove whistespaces which also removes backlines. We should instead merge all doc comments before doing anything.