[DOC][MINOR][SPARKR] Update SparkR doc for names, columns and colnames

Update R doc:
1. columns, names and colnames returns a vector of strings, not **list** as in current doc.
2. `colnames<-` does allow the subset assignment, so the length of `value` can be less than the number of columns, e.g., `colnames(df)[1] <- "a"`.

felixcheung

Author: actuaryzhang <actuaryzhang10@gmail.com>

Closes #17115 from actuaryzhang/sparkRMinorDoc.

R/pkg/R/DataFrame.R

@@ -280,7 +280,7 @@ setMethod("dtypes",
 
 #' Column Names of SparkDataFrame
 #'
-#' Return all column names as a list.
+#' Return a vector of column names.
 #'
 #' @param x a SparkDataFrame.
 #'
@@ -338,7 +338,7 @@ setMethod("colnames",
           })
 
 #' @param value a character vector. Must have the same length as the number
-#'              of columns in the SparkDataFrame.
+#'              of columns to be renamed.
 #' @rdname columns
 #' @aliases colnames<-,SparkDataFrame-method
 #' @name colnames<-

R/pkg/inst/tests/testthat/test_sparkSQL.R

@@ -898,6 +898,12 @@ test_that("names() colnames() set the column names", {
   expect_equal(names(z)[3], "c")
   names(z)[3] <- "c2"
   expect_equal(names(z)[3], "c2")
+
+  # Test subset assignment
+  colnames(df)[1] <- "col5"
+  expect_equal(colnames(df)[1], "col5")
+  names(df)[2] <- "col6"
+  expect_equal(names(df)[2], "col6")
 })
 
 test_that("head() and first() return the correct data", {