Add documentation for use of incoming links in needtable

[fournier-p]

Currently there is no mention in the documentation that we can set a needtable like so:
(let's assume that we have an extra link option called covers)

.. needtable::
   :columns: id, title, covers, covers_back

The table will then render the outgoint links in the third column and incomint links in the fourth column.

I had to go look in the source code to find where this is defined. (link)

This is an extremely useful feature for traceability matrices and should definetely be documented.