Update the documentation policies: Ask to use docstrings to document API members

docs/contributing.rst

@@ -165,18 +165,18 @@ Scrapy:
 Documentation policies
 ======================
 
-* **Don't** use docstrings for documenting classes, or methods which are
-  already documented in the official (sphinx) documentation. Alternatively,
-  **do** provide a docstring, but make sure sphinx documentation uses
-  autodoc_ extension to pull the docstring. For example, the
-  :meth:`ItemLoader.add_value` method should be either
-  documented only in the sphinx documentation (not as a docstring), or
-  it should have a docstring which is pulled to sphinx documentation using
-  autodoc_ extension.
+For reference documentation of API members (classes, methods, etc.) use
+docstrings and make sure that the Sphinx documentation uses the autodoc_
+extension to pull the docstrings. API reference documentation should be
+IDE-friendly: short, to the point, and it may provide short examples.
 
-* **Do** use docstrings for documenting functions not present in the official
-  (sphinx) documentation, such as functions from ``scrapy.utils`` package and
-  its sub-modules.
+Other types of documentation, such as tutorials or topics, should be covered in
+files within the ``docs/`` directory. This includes documentation that is
+specific to an API member, but goes beyond API reference documentation.
+
+In any case, if something is covered in a docstring, use the autodoc_
+extension to pull the docstring into the documentation instead of duplicating
+the docstring in files within the ``docs/`` directory.
 
 .. _autodoc: http://www.sphinx-doc.org/en/stable/ext/autodoc.html