doc: add two similar overlay "side-by-side" examples.

doc/filters.texi

@@ -2367,6 +2367,13 @@ ffmpeg -i input -i logo1 -i logo2 -filter_complex
 # add a transparent color layer on top of the main video,
 # WxH specifies the size of the main input to the overlay filter
 color=red@.3:WxH [over]; [in][over] overlay [out]
+
+# play an original video and a filtered version (here with the deshake filter)
+# side by side
+ffplay input.avi -vf 'split[a][b]; [a]pad=iw*2:ih[src]; [b]deshake[filt]; [src][filt]overlay=w'
+
+# the previous example is the same as:
+ffplay input.avi -vf 'split[b], pad=iw*2[src], [b]deshake, [src]overlay=w'
 @end example
 
 You can chain together more overlays but the efficiency of such