[DOCS] Document HMR virtual site limitation

[IAlibay]

Description

Virtual sites are not currently allowed when doing HMR, however there is no documentation at the "to_openmm_system" docstring level (or elsewhere as far as I can tell). This means you have to encounter the error to know the limitation.

openff-interchange/openff/interchange/interop/openmm/__init__.py

Lines 183 to 187 in 57dd53a

	if system.getNumParticles() != interchange.topology.n_atoms:
	raise UnsupportedExportError(
	"Hydrogen mass repartitioning with virtual sites present, even on "
	" rigid water, is not yet supported.",
	)

The ask

A little bit more docstring docs here:

openff-interchange/openff/interchange/interop/openmm/__init__.py

Lines 56 to 57 in 57dd53a

	The mass to use for hydrogen atoms if not present in the topology. If non-trivially different
	than the default value, mass will be transferred from neighboring heavy atoms.