TMath is a namespace. And as such it doesn’t have one defined location for declarations and definitions (header and source). In fact THtml doesn’t know of any source file that’s relevant for TMath, so it can’t process it.
We will improve the situation with documenting namespaces somewhat over the next few months, but I only expect a fundamental improvement (location of functions across different headers / source files etc) during the next year, when THtml will be updated to use Reflex.