Typo and bug fixes (#124)

* Clearer text in figure caption.

* Wording improvement in Slabs section.

* remove quotes

* comma placement

* slab scale is mesoscopic

* Clarify wording about bulk slabs, following suggestions from François

* diffuse is opaque too

* ambient medium only needed at top, unless thin-walled

* remove brackets

* italics not quotes, for introduced terminology

* Too many parentheses.

* Remove "agnostic" language for EDFs.

* merge

* Anisotropy formatting cleanups.
Thin-film wording.

* Add reference for "volumetric path tracing".

* **`subsurface_radius_scale`** defined above table.
Dispersion section had poor spacing, improved it.

* Add deleted note about future thin-walled model improvements.
Reference to thin-wall figure.

* improve bulk slab description

* more concise

* put back Adrien's wording improvements to bulk slab description

* Added a reference to more up-to-date student-t NDF, since we are mentioning GTR.

* Student's t-distribution, not Student-T

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Co-authored-by: Jonathan Stone <[email protected]>