Improve documentation

[bstaton1]

• consistent text for functions with the same argument
• consistent capitalization and other things
• in post_summ: include a note for long running summaries. Indicate rounding rules for ESS and Rhat
• write a better DESCRIPTION file
• decide on whether to keep "nodes" terminology
• replace "percentile" with "quantile" throughout
• all exported functions must have a return value in help file
• make sure no warnings are returned by documentation (e.g., some links to things like as.mcmc.list are returning warnings)