improve documentation for chi_update_sql()

[dcolombara]

documentation for replace_table parameter is unclear.

Replace with the following:

#' @param replace_table If TRUE, replace the entire table with new data (WARNING: 
#' may permanently delete existing indicator_keys not present in new data). 
#' If FALSE, replace only the rows matching the indicator_keys in the new data 
#' and append any new indicator_keys.
#' 
#' Default \code{replace_table = FALSE}

[dcolombara]

close with d2c798e