[SPARK-21293][SS][SPARKR] Add doc example for streaming join, dedup

## What changes were proposed in this pull request?

streaming programming guide changes

## How was this patch tested?

manually

Author: Felix Cheung <felixcheung_m@hotmail.com>

Closes #20340 from felixcheung/rstreamdoc.

docs/structured-streaming-programming-guide.md

@@ -1100,6 +1100,21 @@ streamingDf.join(staticDf, "type")  # inner equi-join with a static DF
 streamingDf.join(staticDf, "type", "right_join")  # right outer join with a static DF
 {% endhighlight %}
 
+</div>
+
+<div data-lang="r"  markdown="1">
+
+{% highlight r %}
+staticDf <- read.df(...)
+streamingDf <- read.stream(...)
+joined <- merge(streamingDf, staticDf, sort = FALSE)  # inner equi-join with a static DF
+joined <- join(
+            staticDf,
+            streamingDf, 
+            streamingDf$value == staticDf$value,
+            "right_outer")  # right outer join with a static DF
+{% endhighlight %}
+
 </div>
 </div>
 
@@ -1227,6 +1242,30 @@ impressionsWithWatermark.join(
 
 {% endhighlight %}
 
+</div>
+<div data-lang="r"  markdown="1">
+
+{% highlight r %}
+impressions <- read.stream(...)
+clicks <- read.stream(...)
+
+# Apply watermarks on event-time columns
+impressionsWithWatermark <- withWatermark(impressions, "impressionTime", "2 hours")
+clicksWithWatermark <- withWatermark(clicks, "clickTime", "3 hours")
+
+# Join with event-time constraints
+joined <- join(
+  impressionsWithWatermark,
+  clicksWithWatermark,
+  expr(
+    paste(
+      "clickAdId = impressionAdId AND",
+      "clickTime >= impressionTime AND",
+      "clickTime <= impressionTime + interval 1 hour"
+)))
+
+{% endhighlight %}
+
 </div>
 </div>
 
@@ -1287,6 +1326,23 @@ impressionsWithWatermark.join(
 
 {% endhighlight %}
 
+</div>
+<div data-lang="r"  markdown="1">
+
+{% highlight r %}
+joined <- join(
+  impressionsWithWatermark,
+  clicksWithWatermark,
+  expr(
+    paste(
+      "clickAdId = impressionAdId AND",
+      "clickTime >= impressionTime AND",
+      "clickTime <= impressionTime + interval 1 hour"),
+  "left_outer"                 # can be "inner", "left_outer", "right_outer"
+))
+
+{% endhighlight %}
+
 </div>
 </div>
 
@@ -1441,15 +1497,29 @@ streamingDf
 {% highlight python %}
 streamingDf = spark.readStream. ...
 
-// Without watermark using guid column
+# Without watermark using guid column
 streamingDf.dropDuplicates("guid")
 
-// With watermark using guid and eventTime columns
+# With watermark using guid and eventTime columns
 streamingDf \
   .withWatermark("eventTime", "10 seconds") \
   .dropDuplicates("guid", "eventTime")
 {% endhighlight %}
 
+</div>
+<div data-lang="r"  markdown="1">
+
+{% highlight r %}
+streamingDf <- read.stream(...)
+
+# Without watermark using guid column
+streamingDf <- dropDuplicates(streamingDf, "guid")
+
+# With watermark using guid and eventTime columns
+streamingDf <- withWatermark(streamingDf, "eventTime", "10 seconds")
+streamingDf <- dropDuplicates(streamingDf, "guid", "eventTime")
+{% endhighlight %}
+
 </div>
 </div>