merge: make "linear" an input in docstring table
Instead of having two ouputs, it seem simpler to have an addition
input. This will allow further simplification.
merge: drop redundant column in docstring table
The "same" and "cross" columns now have the same values, so let's
combine them into "non-linear".
merge: drop obsolete non-linear cases from docstring table
Since
6b1fc09c699a (update: change default destination to tipmost
descendant (
issue4673) (BC), 2016-02-02), non-linear updates can no
longer happen if the user doesn't specify a destination, so we can
drop these case from the table in the docstring of merge.update().