MCP/API: accept HTML in update_document, like create_document, so documents created from HTML can be updated

Since #12943, create_document accepts format: "html", which keeps structures Markdown can't represent, such as bullet lists inside table cells. update_document and documents.update only accept Markdown, so a document created from HTML can't be updated through the API without losing that structure. What we see on our self-hosted instance: • Our requirement documents, migrated from Confluence, have tables whose cells contain nested bullet lists. • Any Markdown update that touches a table, even a patch that changes a single word, re-parses the whole table from Markdown. This flattens the lists in every cell and drops the spaces around bold and strikethrough text. The rest of the document is fine. • The only API workaround is to delete the document and recreate it from HTML. That changes its URL and loses its history and comments, and our Jira tickets link to these URLs. • For now we apply changes by hand in the editor, where pasting HTML…