Clean up r2mlm_long documentation

[mkshaw]

Better example of the documentation:

r2mlm_long_manual(
  data = test,
  covs = c("salary_c", "salary_m", "s_t_ratio", "interaction"),
  random_covs = c("salary_c"),
  clusterID = "schoolID",
  gammas = c(0.0771823, 0.0384119, -0.0399241, -0.0001657),
  Tau = matrix(c(0.4244790288, 0.0005614634, 
                 0.0005614634, 0.0022967152), 2, 2),
  sigma2 = 0.826907,
  bargraph = T
)