added Rd documentation for RW_multinomial sampler
added Rd documentation for RW_multinomial sampler

nimble-dev/nimble,NLMichaud/nimble,nxdao2000/nimble,nimble-dev/nimble,NLMichaud/nimble,NLMichaud/nimble,nxdao2000/nimble,nxdao2000/nimble,NLMichaud/nimble,nimble-dev/nimble,nimble-dev/nimble,nxdao2000/nimble,nimble-dev/nimble