ir: Honor documentation_length in Cython.

tests/expectations/doclength_short.pyx

@@ -7,14 +7,7 @@ cdef extern from *:
 cdef extern from *:
 
   # The root of all evil.
-  #
-  # But at least it contains some more documentation as someone would expect
-  # from a simple test case like this. Though, this shouldn't appear in the
-  # output.
   void root();
 
   # A little above the root, and a lot more visible, with a run-on sentence
-  # to test going over the first line.
-  #
-  # Still not here, though.
   void trunk();