lavfi/split: fix minor documentation issues

Use verbal form to describe "outputs" options, and mention it in the
manual.

doc/filters.texi

@@ -5727,8 +5727,8 @@ subtitles=filename=sub.srt
 
 Split input video into several identical outputs.
 
-The filter accepts a single parameter which specifies the number of outputs. If
-unspecified, it defaults to 2.
+The filter accepts a single option @option{outputs} which specifies
+the number of outputs. If unspecified, it defaults to 2.
 
 For example
 @example

libavfilter/split.c

@@ -93,7 +93,7 @@ static int filter_frame(AVFilterLink *inlink, AVFrame *frame)
 #define OFFSET(x) offsetof(SplitContext, x)
 #define FLAGS AV_OPT_FLAG_AUDIO_PARAM | AV_OPT_FLAG_VIDEO_PARAM
 static const AVOption options[] = {
-    { "outputs", "Number of outputs", OFFSET(nb_outputs), AV_OPT_TYPE_INT, { .i64 = 2 }, 1, INT_MAX, FLAGS },
+    { "outputs", "set number of outputs", OFFSET(nb_outputs), AV_OPT_TYPE_INT, { .i64 = 2 }, 1, INT_MAX, FLAGS },
     { NULL },
 };