Actual mines are very three-dimensional and rarely, if ever, in neat "levels" like a dungeon mapped out on graph paper. A three-dimensional model is usually much clearer than any flat representation could be. Notes would include not just lengths, widths, and heights, but a lot of angular measurements of turning, ascent, and descent. Sketches would likely be less of the layout than of particular features, such as carvings or fossils. Dwarves are typically portrayed as having living areas dug in near the actually mining, so this area could easily be level and orderly, easily mapped on a flat surface. There could be geological notes (including sites promising for future mining) and possibly a cross-sectional illustration of the various layers of rock at the mine location. There could be some Dwarven "dad jokes" * in the notes as the fellow kept himself amused.
* - possibly incomprehensible to a non-Dwarf, at least one without a good knowledge of Dwarven culture and/or mining technique and/or geology.