Enhance Superelevation documentation with details

[RickBrice]

Added details about positive and negative superelevation, including a reference to the IfcProfileDef coordinate system and an image illustrating the concept.

[evandroAlfieri]

thanks, we'll process this as part of other editorial improvements for 4.3

Just fyi, and for future reference, images are treated differently.

See for example IfcSectionedSurface

See how the maintainance of a unique, ordered and consistent numbering of figures across the spec is achieved, while having a self-contained markdown.

Figure 2 illustrates an _IfcSectionedSurface_ featuring branching longitudinal breaklines.

![sectioned surface](../../../../figures/IfcSectionedSurface_2.png)

Figure 2 — Sectioned surface with branching longitudinal breaklines

As long as the figure is correctly placed and referenced, the system does the job of numbering.