drawtext: extend/clarify examples in the filter description

doc/filters.texi

@@ -464,12 +464,22 @@ Default value is 4.
 
 For example the command:
 @example
-drawtext="fontfile=FreeSerif.ttf: text='Test Text': x=100: y=50: fontsize=24: fontcolor=yellow@@0.2: boxcolor=red@@0.2: box=1"
+drawtext="fontfile=/usr/share/fonts/truetype/freefont/FreeSerif.ttf: text='Test Text'"
 @end example
 
-will draw 'Test Text' with font FreeSerif of size 24 at position
-(100,50), text color is yellow, and draw a red box around text. Both
-the text and the box have an opacity of 20%.
+will draw "Test Text" with font FreeSerif, using the default values
+for the optional parameters.
+
+The command:
+@example
+drawtext="fontfile=/usr/share/fonts/truetype/freefont/FreeSerif.ttf: text='Test Text':\
+          x=100: y=50: fontsize=24: fontcolor=yellow@@0.2: box=1: boxcolor=red@@0.2"
+@end example
+
+will draw 'Test Text' with font FreeSerif of size 24 at position x=100
+and y=50 (counting from the top-left corner of the screen), text is
+yellow with a red box around it. Both the text and the box have an
+opacity of 20%.
 
 Note that the double quotes are not necessary if spaces are not used
 within the parameter list.