Expanded and improved the docstring for the relax_disp.r20_from_min_r2eff user function.
This follows from http://thread.gmane.org/gmane.science.nmr.relax.devel/5957.
The documentation now covers a number of the uses for this user function. The text has also been
lightly edited. To fit all the text into the GUI user function window, the size of the dialog and
the text high settings have been changed.
