The hand-written classes should have documentation

The classes and functions in e.g. `include/vpi_uhdm.h` should have some documentation headers to understand the context and purpse.

Start by reviewing the hand-written declarations in include/vpi_uhdm.h and identify the classes and functions that lack documentation headers. Done means the relevant public classes and functions have headers explaining their context and purpose; inspect nearby declarations for the project's existing documentation style.

Written by the indexing model from the issue text.