Improve documentation for 3D primitives.

[player-03]

• Using "section" instead of "segment" helps avoid confusion with LineSegment. This is mostly important for wireframe primitives, but I changed everything for consistency.
• Describing the shape as being "divided into sections" along an axis helps make it clearer what the sections actually represent. It's only one data point, but I found it easy to interpret the old documentation backwards.
• I described round shapes in terms of circumference and height, rather than width and height, because width could be interpreted as not wrapping around the shape.
• I fixed some typos and incorrect descriptions.

[player-03]

An example of what I found confusing:

On the left, the sections are vertical but correspond with the X axis (or circumference). On the right, the sections are horizontal but correspond with the Y axis.

The numbers indicate what's actually being counted: the number of times the section fits into the cylinder. sectionsC is 8, because the circumference is divided into 8 of those vertical sections. sectionsH is 4, because the height is divided into 4 of those ring-shaped sections. Hopefully, the new wording makes all this clearer, so no one else makes the same mistake I did.