footnote: find_containing_document function improvment for platform…

… parser and some additional comments

docx/oxml/text/run.py

@@ -83,6 +83,8 @@ def clear_content(self):
         """
         content_child_elms = self[1:] if self.rPr is not None else self[:]
         for child in content_child_elms:
+            # We keep ``w:footnoteReference`` because of the
+            # platform `replace_special_chars_preprocessor` preprocessor.
             if child.tag == qn('w:footnoteReference'):
                 continue
             self.remove(child)

docx/shared.py

@@ -286,12 +286,20 @@ def find_containing_document(element):
     """
     from .document import Document
     while True:
-        if not hasattr(element, '_parent'):
-            raise PythonDocxError(f'{type(element)} has no `_parent` property.')
-        if isinstance(element._parent, Document):
-            return element._parent
+        if hasattr(element, '_document_part'):
+            return element._document_part.document
+        elif hasattr(element, '_parent'):
+            if isinstance(element._parent, Document):
+                return element._parent
+            else:
+                element = element._parent
+        elif hasattr(element, 'parent'):
+            if isinstance(element.parent, Document):
+                return element.parent
+            else:
+                element = element.parent
         else:
-            element = element._parent
+            raise PythonDocxError(f'{type(element)} couldn\'t find root Document.')
 
 
 def is_valid_url(url):

docx/text/paragraph.py

@@ -780,7 +780,7 @@ def image_parts(self):
         """
         Return all image parts related to this paragraph.
         """
-        doc = self.part.document
+        doc = find_containing_document(self)
         drawings = []
         parts = []