Fix "Expected an item" error when mixing doc-comments and other attributes

[ironcev]

Doc-comments are attributes and can be arbitrarily mixed with other attributes. E.g., the following example should compile but it produces an "Expected an item" error:

/// This is the first line of the comment.
#[allow(dead_code)]
/// This is the second line.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Expected an item.
pub fn fun() {}

The resulting documentation string in forc doc and LSP must be the joint of all the doc comments.