Document mr.calcRfCenter with structured help block

[rmoskwa]

Echoing your comment from PR #238 here, so there is a record. I did NOT implement a change for your comment so that there was no silent change, and also I think it would be best if the update came from you as the expert.

in the documentation of calcRfCenter() you write:

%       Returned tc is on the RF raster (rf.t edges); fi captures the
%       sub-raster offset when rf.center does not coincide with a sample.

with which I disagree; tc is just the time between the beginning of the shape (the front edge of the firs RF sample, not its center) and the actual center of the RF shape. It is not aligned to the RF raster in any way.