Split off from #62855
Currently, rustdoc ignores any doc comments found on 'pub use'
statements. As described in issue #58700, this makes it impossible to
properly document procedural macros. Any doc comments must be written on
the procedural macro definition, which must occur in a dedicated
proc-macro crate. This means that any doc comments or doc tests cannot
reference items defined in re-exporting crate, despite the fact that
such items may be required to use the procedural macro.
To solve this issue, this commit allows doc comments to be written on
'pub use' statements. For consistency, this applies to *all* 'pub use'
statements, not just those importing procedural macros.
When inlining documentation, documentation on 'pub use' statements will
be prepended to the documentation of the inlined item. For example,
the following items:
```rust
mod other_mod {
/// Doc comment from definition
pub struct MyStruct;
}
/// Doc comment from 'pub use'
///
pub use other_mod::MyStruct;
```
will caues the documentation for the re-export of 'MyStruct' to be
rendered as:
```
Doc comment from 'pub use'
Doc comment from definition
```
Note the empty line in the 'pub use' doc comments - because doc comments
are concatenated as-is, this ensure that the doc comments on the
definition start on a new line.
