improve the documentation on accessors

doc/internals.rst

@@ -116,6 +116,10 @@ xarray:
 
 .. literalinclude:: examples/_code/accessor_example.py
 
+In general, the only restriction on the accessor class is that the ``__init__`` method
+must have a single parameter: the ``Dataset`` or ``DataArray`` object it is supposed
+to work on.
+
 This achieves the same result as if the ``Dataset`` class had a cached property
 defined that returns an instance of your class:
 
@@ -173,9 +177,9 @@ for an open source library. In the future, we will maintain a list of accessors
 and the libraries that implement them on this page.
 
 To make documenting accessors with ``sphinx`` and ``sphinx.ext.autosummary``
-easier, you can use `sphinx-ext-autosummary`_.
+easier, you can use `sphinx-autosummary-accessors`_.
 
-.. _sphinx-ext-autosummary: https://sphinx-autosummary-accessors.readthedocs.io/
+.. _sphinx-autosummary-accessors: https://sphinx-autosummary-accessors.readthedocs.io/
 
 .. _zarr_encoding:
 
@@ -226,4 +230,4 @@ re-open it directly with Zarr:
 
     zgroup = zarr.open("rasm.zarr")
     print(zgroup.tree())
-    dict(zgroup["Tair"].attrs)
+    dict(zgroup["Tair"].attrs)

doc/whats-new.rst

@@ -35,6 +35,8 @@ Bug fixes
 
 Documentation
 ~~~~~~~~~~~~~
+- add information about requirements for accessor classes (:issue:`2788`, :pull:`4657`).
+  By `Justus Magin <https://github.com/keewis>`_.
 - start a list of external I/O integrating with ``xarray`` (:issue:`683`, :pull:`4566`).
   By `Justus Magin <https://github.com/keewis>`_.