remove udf todo's

we had quite bad-looking todo's for stuff that is actually not used that much in practice

docs/udf.rst

@@ -187,7 +187,8 @@ which is the modus operandi of the ``apply`` processes.
 
     In practice it will be a lot easier and more efficient to do this kind of rescaling
     with pre-defined openEO math processes, for example: ``s2_cube.apply(lambda x: 0.0001 * x)``.
-    This is just a very simple illustration to get started with UDFs.
+    This is just a very simple illustration to get started with UDFs. In fact, it's very likely that
+    you will never want to use a UDF with apply.
 
 UDF script
 ----------
@@ -316,15 +317,12 @@ To invoke a UDF like this, the apply_neighborhood method is most suitable:
         ], overlap=[])
 
 
-Illustration of data chunking in ``apply`` with a  UDF
-========================================================
-
-TODO
 
 Example: ``apply_dimension`` with a UDF
 ========================================
 
-TODO
+This is useful when running custom code over all band values for a given pixel or all observations per pixel.
+See section below 'Smoothing timeseries with a user defined function' for a concrete example.
 
 Example: ``reduce_dimension`` with a UDF
 ========================================