Improve docs on markdown docs in DAGs (apache#42012)

docs/apache-airflow/core-concepts/dags.rst

@@ -653,8 +653,8 @@ doc_md      markdown
 doc_rst     reStructuredText
 ==========  ================
 
-Please note that for DAGs, ``doc_md`` is the only attribute interpreted. For DAGs it can contain a string or the reference to a template file. Template references are recognized by str ending in ``.md``.
-If a relative path is supplied it will start from the folder of the DAG file. Also the template file must exist or Airflow will throw a ``jinja2.exceptions.TemplateNotFound`` exception.
+Please note that for DAGs, ``doc_md`` is the only attribute interpreted. For DAGs it can contain a string or the reference to a markdown file. Markdown files are recognized by str ending in ``.md``.
+If a relative path is supplied it will be loaded from the path relative to which the Airflow Scheduler or DAG parser was started. If the markdown file does not exist, the passed filename will be used as text, no exception will be displayed. Note that the markdown file is loaded during DAG parsing, changes to the markdown content take one DAG parsing cycle to have changes be displayed.
 
 This is especially useful if your tasks are built dynamically from configuration files, as it allows you to expose the configuration that led to the related tasks in Airflow: