Add some documentation within bezier_curve.cc

[danieldouglas92]

A little more documentation could probably be added around the section of code where the side of the line that each control point lies on is calculated, but I wanted to open this for now to get feedback on what I wrote.

[github-actions]

Benchmark	Main	Feature	Difference (99.9% CI)
Slab interpolation simple none	1.117 ± 0.011 (s=412)	1.117 ± 0.009 (s=396)	-0.2% .. +0.2%
Slab interpolation curved simple none	1.123 ± 0.011 (s=407)	1.124 ± 0.012 (s=397)	-0.1% .. +0.4%
Spherical slab interpolation simple none	1.095 ± 0.009 (s=422)	1.095 ± 0.008 (s=402)	-0.2% .. +0.2%
Slab interpolation simple curved CMS	1.163 ± 0.010 (s=424)	1.163 ± 0.011 (s=353)	-0.2% .. +0.2%
Spherical slab interpolation simple CMS	1.442 ± 0.012 (s=305)	1.442 ± 0.012 (s=322)	-0.2% .. +0.2%
Spherical fault interpolation simple none	1.101 ± 0.009 (s=391)	1.101 ± 0.008 (s=429)	-0.2% .. +0.1%
Cartesian min max surface	2.540 ± 0.032 (s=182)	2.537 ± 0.016 (s=175)	-0.5% .. +0.2%
Spherical min max surface	7.180 ± 0.072 (s=63)	7.170 ± 0.079 (s=65)	-0.8% .. +0.5%

[MFraters]

Thanks!