Improve survey-utils docs

From Katie's suggestions in
cmu-delphi/covidcast-indicators#645

Author: capnrefsmmat

docs/symptom-survey/survey-utils.R

@@ -5,14 +5,14 @@ library(dplyr)
 #' Fetch all survey data in a chosen directory.
 #'
 #' There can be multiple data files for a single day of survey responses, for
-#' example if the data is reissued when late-arriving surveys are recorded.
-#' Each file contains *all* data recorded for that date, so only the later files
-#' are needed.
+#' example if the data is reissued when late-arriving surveys are recorded. Each
+#' file contains *all* data recorded for that date, so only the most recently
+#' updated file for each date is needed.
 #'
-#' This function extracts the date from each file, determines which files are
-#' reissued data, and produces a single data frame representing the most recent
-#' data available for each day. It can read gzip-compressed CSV files, such as
-#' those on the SFTP site, using `readr::read_csv`.
+#' This function extracts the date from each file, determines which files
+#' contain reissued data, and produces a single data frame representing the most
+#' recent data available for each day. It can read gzip-compressed CSV files,
+#' such as those on the SFTP site, using `readr::read_csv`.
 #'
 #' This function handles column types correctly for surveys up to Wave 4.
 #'