Draft: docs: use proper SI-units for sizes

In SI units, K means 10^3, M means 10^6 and G means 10^9. While most programmers will understand that we're referring to the 1024-based units, we can be clearer here.

So let's use the proper SI prefixes here instead, to remove any chance of confusion. While we're at it, let's also add a space between the number and the unit in the cases where that was missing.