Improve documentation of nsim_obs and obs_prob in likelihood()

[jamesmbaazam]

obs_prob currently says "Observation probability (assumed constant)". It could be improved to add examples and their interpretation, for example, "Observation probability. A value between 0 and 1. Any value below 1 implies imperfect observation and nsim_obs will be required. In the simulation, the observation process is assumed to be constant throughout the epidemic."

nsim_obs currently says "Number of simulations if the log-likelihood/likelihood is to be approximated for imperfect observation" but can be improved to say "Number of simulations if the log-likelihood/likelihood is to be approximated for imperfect observation, that is if obs_prob is below 1."

This way, it links the two arguments and helps to partly address #125 by clarifying which arguments are required and when.