Function suggest_insertion 

fn suggest_insertion(
    cx: &DocContext<'_>,
    item: &Item,
    dox: &str,
    lint: &mut Diag<'_, ()>,
    insert_index: usize,
    suggestion: char,
    message: &'static str,
)

Try to emit a span suggestion and fall back to help messages if we can’t find a suitable span.

This helps finding backticks in huge macro-generated docs.