Clarify Segment Endpoints in Libgeometry

[MaxineK36]

While the header files in the libgeometry library clearly specify which point arguments to segment_intersect correspond to which segments, students looking only at the .c file often assume that the two segments being checked for intersection are (p1, p2) and (q1, q2) when in reality the two segments are (p1, q1) and (p2, q2). The code should be updated with additional comments to clarify, so students don't lose time on this confusion.