Clarify the use of the interl=1|-1 option of the scale filter.

It is not entirely clear that whilst for width and height only an
expression needs to be provided, for interlace the option must
also be given.

It is also unclear that the default is non interlaced aware scaling.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/filters.texi

@@ -2280,6 +2280,8 @@ select interlaced aware scaling depending on whether the source frames
 are flagged as interlaced or not
 @end table
 
+Unless @var{interl} is set to one of the above options, interlaced scaling will not be used.
+
 Some examples follow:
 @example
 # scale the input video to a size of 200x100.
@@ -2290,6 +2292,9 @@ scale=2*iw:2*ih
 # the above is the same as
 scale=2*in_w:2*in_h
 
+# scale the input to 2x with forced interlaced scaling
+scale=2*iw:2*ih:interl=1
+
 # scale the input to half size
 scale=iw/2:ih/2