Add more docmentation for get_first_sdpor_initial()

Add more docmentation for get_first_sdpor_initial()

A very important function at the center of SDPOR
was missing significant context that would help
a reader understand what exactly was going on
with the function. What makes the situation tricky
is the fact that the pseudocode's implementation
is blended into several lines and computed "on the
fly" rather than fully.