doc/filters: add drawtext example

Show example which draws text at a random position, and switches positions every 30 seconds
Signed-off-by: Mulvya <mulvya@gmail.com>
Signed-off-by: Lou Logan <lou@lrcd.com>

doc/filters.texi

@@ -6556,6 +6556,12 @@ Show the text at the center of the video frame:
 drawtext="fontsize=30:fontfile=FreeSerif.ttf:text='hello world':x=(w-text_w)/2:y=(h-text_h)/2"
 @end example
 
+@item
+Show the text at a random position, switching to a new position every 30 seconds:
+@example
+drawtext="fontsize=30:fontfile=FreeSerif.ttf:text='hello world':x=if(eq(mod(t\,30)\,0)\,rand(0\,(w-text_w))\,x):y=if(eq(mod(t\,30)\,0)\,rand(0\,(h-text_h))\,y)"
+@end example
+
 @item
 Show a text line sliding from right to left in the last row of the video
 frame. The file @file{LONG_LINE} is assumed to contain a single line