Document checks for units consistency in component math

[nickerso]

As part of #818 there is some comments in the implementation of Analyser::AnalyserImpl::analyseEquationUnits that describes the requirements for units consistency in the component math. This is useful information that should be surfaced in the libCellML documentation and/or website somewhere.

This ticket is to remind us to decide where that should be and how to get it there.