Document: _namespace is a reserved field for DoFns (used for display data)

[damccorm]

User reported issue when _namespace is not used for display data:

> self._populate_items(display_data_dict)
> File third_party/py/apache_beam/transforms/display.py", line 117, in _populate_items
> element._namespace(), element.display_data())
> TypeError: 'str' object is not callable

Imported from Jira BEAM-12340. Original Jira may contain additional context.
Reported by: altay.

[svetakvsundhar]

@tvalentyn ; could you provide more context on this issue? Looks like we should document this here [1] if IIUC?

[1]
https://github.com/apache/beam/blob/master/website/www/site/content/en/documentation/transforms/python/elementwise/pardo.md

[tvalentyn]

I think we can rename HasDisplayData._namespace() to HasDisplayData._get_display_data_namespace() to prevent such collisions. As far as I can tell, the changes will be contained to display.py.