Update documentation to midwest variables (#4274)

* Add periods to midwest variables for consistency

* Add reference for midwest data

See https://git.io/JkSOi (ggplot2 book).

* Add missing midwest variable descriptions

* Fix variable comment for population to density

* Add user note for using midwest dataset

* Use canonical form of CRAN acs package link

* Document

Co-authored-by: GitHub Actions <actions@github.com>

Author: erictleung

R/data.R

@@ -48,16 +48,21 @@
 
 #' Midwest demographics
 #'
-#' Demographic information of midwest counties
+#' Demographic information of midwest counties from 2000 US census
+#'
+#' Note: this dataset is included for illustrative purposes. The original
+#' descriptions were not documented and the current descriptions here are based
+#' on speculation. For more accurate and up-to-date US census data, see the
+#' [`acs` package](https://cran.r-project.org/package=acs).
 #'
 #' @format A data frame with 437 rows and 28 variables:
 #' \describe{
-#'  \item{PID}{}
-#'  \item{county}{}
-#'  \item{state}{}
-#'  \item{area}{}
-#'  \item{poptotal}{Total population}
-#'  \item{popdensity}{Population density}
+#'  \item{PID}{Unique county identifier.}
+#'  \item{county}{County name.}
+#'  \item{state}{State to which county belongs to.}
+#'  \item{area}{Area of county (units unknown).}
+#'  \item{poptotal}{Total population.}
+#'  \item{popdensity}{Population density (person/unit area).}
 #'  \item{popwhite}{Number of whites.}
 #'  \item{popblack}{Number of blacks.}
 #'  \item{popamerindian}{Number of American Indians.}
@@ -69,17 +74,17 @@
 #'  \item{percasian}{Percent Asian.}
 #'  \item{percother}{Percent other races.}
 #'  \item{popadults}{Number of adults.}
-#'  \item{perchsd}{}
+#'  \item{perchsd}{Percent with high school diploma.}
 #'  \item{percollege}{Percent college educated.}
-#'  \item{percprof}{Percent profession.}
-#'  \item{poppovertyknown}{}
-#'  \item{percpovertyknown}{}
-#'  \item{percbelowpoverty}{}
-#'  \item{percchildbelowpovert}{}
-#'  \item{percadultpoverty}{}
-#'  \item{percelderlypoverty}{}
-#'  \item{inmetro}{In a metro area.}
-#'  \item{category}{}
+#'  \item{percprof}{Percent with professional degree.}
+#'  \item{poppovertyknown}{Population with known poverty status.}
+#'  \item{percpovertyknown}{Percent of population with known poverty status.}
+#'  \item{percbelowpoverty}{Percent of people below poverty line.}
+#'  \item{percchildbelowpovert}{Percent of children below poverty line.}
+#'  \item{percadultpoverty}{Percent of adults below poverty line.}
+#'  \item{percelderlypoverty}{Percent of elderly below poverty line.}
+#'  \item{inmetro}{County considered in a metro area.}
+#'  \item{category}{Miscellaneous.}
 #' }
 #'
 "midwest"