yadif: correct documentation on the parity parameter

0 is top-field-first, 1 is bottom-field-first, not the other way
around.

Fix trac issue #380.

doc/filters.texi

@@ -1846,9 +1846,9 @@ interlaced video, accepts one of the following values:
 
 @table @option
 @item 0
-assume bottom field first
-@item 1
 assume top field first
+@item 1
+assume bottom field first
 @item -1
 enable automatic detection
 @end table

libavfilter/vf_yadif.c

@@ -36,8 +36,8 @@ typedef struct {
     int mode;
 
     /**
-     *  0: bottom field first
-     *  1: top field first
+     *  0: top field first
+     *  1: bottom field first
      * -1: auto-detection
      */
     int parity;