manpage: slap "do not use" label on vf_vapoursynth

Plus some other minor corrections.

DOCS/man/vf.rst

@@ -395,6 +395,11 @@ Available mpv-only filters are:
     of that will return errors. As such, you can't use the full power of
     VapourSynth, but you can use certain filters.
 
+    .. warning::
+
+        Do not use this filter, unless you have expert knowledge in VapourSynth,
+        and know how to fix bugs in the mpv VapourSynth wrapper code.
+
     If you just want to play video generated by VapourSynth (i.e. using
     a native VapourSynth video source), it's better to use ``vspipe`` and a
     pipe or FIFO to feed the video to mpv. The same applies if the filter script
@@ -510,9 +515,10 @@ Available mpv-only filters are:
     The following ``.vpy`` script variables are defined by mpv:
 
     ``video_in``
-        The mpv video source as vapoursynth clip. Note that this has no length
-        set, which confuses many filters. Using ``Trim`` on the clip with a
-        high dummy length can turn it into a finite clip.
+        The mpv video source as vapoursynth clip. Note that this has an
+        incorrect (very high) length set, which confuses many filters. This is
+        necessary, because the true number of frames is unknown. You can use the
+        ``Trim`` filter on the clip to reduce the length.
 
     ``video_in_dw``, ``video_in_dh``
         Display size of the video. Can be different from video size if the
@@ -522,7 +528,7 @@ Available mpv-only filters are:
         FPS value as reported by file headers. This value can be wrong or
         completely broken (e.g. 0 or NaN). Even if the value is correct,
         if another filter changes the real FPS (by dropping or inserting
-        frames), the value of this variable might not be useful. Note that
+        frames), the value of this variable will not be useful. Note that
         the ``--fps`` command line option overrides this value.
 
         Useful for some filters which insist on having a FPS.