Add definition docs (when hovering/autocompleting) for field-like elements (e.g., `math.equation`, `table.cell` etc.)

[Andrew15-5]

Motivation

The more elements supporting the definition docs, the better the UX/DX.

Description

All element functions have definition docs on hover, but not the math.equation. https://typst.app/docs/reference/math/equation/#parameters should be added to the top of the docs (or fully replace the current docs).

#set math.equation()

The same goes to things like table.cell, table.hline etc.

Examples/Questions

Here is what comes up when completing the math.equation or hovering over it: