[SPARK-11294][SPARKR] Improve R doc for read.df, write.df, saveAsTable

Add examples for read.df, write.df; fix grouping for read.df, loadDF; fix formatting and text truncation for write.df, saveAsTable.

Several text issues:
![image](https://cloud.githubusercontent.com/assets/8969467/10708590/1303a44e-79c3-11e5-854f-3a2e16854cd7.png)
- text collapsed into a single paragraph
- text truncated at 2 places, eg. "overwrite: Existing data is expected to be overwritten by the contents of error:"

shivaram

Author: felixcheung <felixcheung_m@hotmail.com>

Closes #9261 from felixcheung/rdocreadwritedf.

R/pkg/R/DataFrame.R

@@ -1572,18 +1572,17 @@ setMethod("except",
 #' spark.sql.sources.default will be used.
 #'
 #' Additionally, mode is used to specify the behavior of the save operation when
-#' data already exists in the data source. There are four modes:
-#'  append: Contents of this DataFrame are expected to be appended to existing data.
-#'  overwrite: Existing data is expected to be overwritten by the contents of
-#     this DataFrame.
-#'  error: An exception is expected to be thrown.
+#' data already exists in the data source. There are four modes: \cr
+#'  append: Contents of this DataFrame are expected to be appended to existing data. \cr
+#'  overwrite: Existing data is expected to be overwritten by the contents of this DataFrame. \cr
+#'  error: An exception is expected to be thrown. \cr
 #'  ignore: The save operation is expected to not save the contents of the DataFrame
-#     and to not change the existing data.
+#'     and to not change the existing data. \cr
 #'
 #' @param df A SparkSQL DataFrame
 #' @param path A name for the table
 #' @param source A name for external data source
-#' @param mode One of 'append', 'overwrite', 'error', 'ignore'
+#' @param mode One of 'append', 'overwrite', 'error', 'ignore' save mode
 #'
 #' @rdname write.df
 #' @name write.df
@@ -1596,6 +1595,7 @@ setMethod("except",
 #' path <- "path/to/file.json"
 #' df <- jsonFile(sqlContext, path)
 #' write.df(df, "myfile", "parquet", "overwrite")
+#' saveDF(df, parquetPath2, "parquet", mode = saveMode, mergeSchema = mergeSchema)
 #' }
 setMethod("write.df",
           signature(df = "DataFrame", path = "character"),
@@ -1637,18 +1637,17 @@ setMethod("saveDF",
 #' spark.sql.sources.default will be used.
 #'
 #' Additionally, mode is used to specify the behavior of the save operation when
-#' data already exists in the data source. There are four modes:
-#'  append: Contents of this DataFrame are expected to be appended to existing data.
-#'  overwrite: Existing data is expected to be overwritten by the contents of
-#     this DataFrame.
-#'  error: An exception is expected to be thrown.
+#' data already exists in the data source. There are four modes: \cr
+#'  append: Contents of this DataFrame are expected to be appended to existing data. \cr
+#'  overwrite: Existing data is expected to be overwritten by the contents of this DataFrame. \cr
+#'  error: An exception is expected to be thrown. \cr
 #'  ignore: The save operation is expected to not save the contents of the DataFrame
-#     and to not change the existing data.
+#'     and to not change the existing data. \cr
 #'
 #' @param df A SparkSQL DataFrame
 #' @param tableName A name for the table
 #' @param source A name for external data source
-#' @param mode One of 'append', 'overwrite', 'error', 'ignore'
+#' @param mode One of 'append', 'overwrite', 'error', 'ignore' save mode
 #'
 #' @rdname saveAsTable
 #' @name saveAsTable

R/pkg/R/SQLContext.R

@@ -452,14 +452,21 @@ dropTempTable <- function(sqlContext, tableName) {
 #'
 #' @param sqlContext SQLContext to use
 #' @param path The path of files to load
-#' @param source the name of external data source
+#' @param source The name of external data source
+#' @param schema The data schema defined in structType
 #' @return DataFrame
+#' @rdname read.df
+#' @name read.df
 #' @export
 #' @examples
 #'\dontrun{
 #' sc <- sparkR.init()
 #' sqlContext <- sparkRSQL.init(sc)
-#' df <- read.df(sqlContext, "path/to/file.json", source = "json")
+#' df1 <- read.df(sqlContext, "path/to/file.json", source = "json")
+#' schema <- structType(structField("name", "string"),
+#'                      structField("info", "map<string,double>"))
+#' df2 <- read.df(sqlContext, mapTypeJsonPath, "json", schema)
+#' df3 <- loadDF(sqlContext, "data/test_table", "parquet", mergeSchema = "true")
 #' }
 
 read.df <- function(sqlContext, path = NULL, source = NULL, schema = NULL, ...) {
@@ -482,9 +489,8 @@ read.df <- function(sqlContext, path = NULL, source = NULL, schema = NULL, ...)
   dataFrame(sdf)
 }
 
-#' @aliases loadDF
-#' @export
-
+#' @rdname read.df
+#' @name loadDF
 loadDF <- function(sqlContext, path = NULL, source = NULL, schema = NULL, ...) {
   read.df(sqlContext, path, source, schema, ...)
 }