docs: clarify myst import dialect role

Author: peter-gy

marimo/_convert/markdown/flavor/mystmd.py

@@ -53,6 +53,8 @@
 
 
 class _MystmdMarkdownImportDialect:
+    """Normalize MyST marimo directives into canonical marimo markdown."""
+
     name: MarkdownFlavorName = "mystmd"
 
     def matches(self, text: str, filepath: str | None) -> bool: