Support doc comments on submodule (mod) items #6879

[ironcev]

Currently, all attributes are forbidden on module items, including doc comments (///). E.g., the following is not allowed:

/// This is an outer doc comment for `some_module`.
mod some_module;

This is unnecessary too restrictive and it breaks the semantic of having both inner and outer attributes and the possibility to combine them. In Rust, if both outer and inner doc comments are provided for a module they get combined in the doc for that modules, following the semantic of combining inner and outer attributes.