Comment parsing: Specify argument numbers for some block commands

The command traits have a member NumArgs for which all the parsing
infrastructure is in place, but no command was setting it to a value
other than 0. By doing so we get warnings when passing an empty
paragraph to \retval (the first argument is the return value, then comes
the description). We also take \xrefitem along for the ride, although as
the documentation states it's unlikely to be used directly.

Reviewed By: gribozavr2

Differential Revision: https://reviews.llvm.org/D125422