Allow MarkupText in revision notes

Currently, the schema dictates that `` contents are limited to text. In practice, there are a few revisions containing multiple paragraphs or newlines for bulleted lists. In principle I don't see any downside to allowing markup.

Locate where the project defines validation for `` content (schema/DTD/XSD and parser/serializer logic), since the request is to allow markup instead of plain text. Confirm whether revision notes are also sanitized or transformed later in the import flow and update that path consistently. Run metadata/validation tests or equivalent schema checks; done means revisions with multi-paragraph or bulleted content are accepted and preserved correctly.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.