doc/scaler: mention default scaling algorithm

Default is "bicubic".
Signed-off-by: Lou Logan <lou@lrcd.com>
Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>

doc/scaler.texi

@@ -13,7 +13,8 @@ FFmpeg tools. For programmatic use, they can be set explicitly in the
 @anchor{sws_flags}
 @item sws_flags
 Set the scaler flags. This is also used to set the scaling
-algorithm. Only a single algorithm should be selected.
+algorithm. Only a single algorithm should be selected. Default
+value is @samp{bicubic}.
 
 It accepts the following values:
 @table @samp