improve target_[xy] documentation

[adriaanph]

current documentation in dataset.py does not state the sign convention adopted for target_x & target_y. consider adding something like one of the following:

a) "Conventions for target_x & target_y are described in katpoint.projection.", or

b) "The (target_x, target_y) coordinates correspond to the (L, M) direction cosines calculated in [Gre1993a]_ and
[Gre1993b]_." (adapted from katpoint.projection), or

c) "The target_y coordinate axis in the plane points along the target's meridian of longitude towards the positive singularity of the sphere (in the direction of increasing elevation or declination, depending on coordinate system used). The target_x coordinate axis is perpendicular to it and points in the direction of increasing latitude (azimuth or right ascension)." (adapted from katpoint.projection).

it might be useful to add simple statements like:
"The coordinate convention roughly translates to [antenna pointing]_x ~ [target centre]_x + [offset]target_x"
"The coordinate convention roughly translates to [antenna pointing]_y ~ [target centre]_y + [offset]target_y"

[adriaanph]

@ludwigschwardt