close #24378
1. Update the start position of span in `nested_doc_comment` correctly.
and hence the spans of identifiers of haddoc can be computed correctly.
2. add test `HaddockSpanIssueT24378`.
close #24378
1. Update the start position of span in `nested_doc_comment` correctly.
and hence the spans of identifiers of haddoc can be computed correctly.
2. add test `HaddockSpanIssueT24378`.
