Revert "manpage: clarify that you can prefetch m3u8 urls"

This commit is based on incorrect information, see previous commit.
It also deletes the correct information the prefetch starts when the
current file is fully load.

The commit also misunderstood the comment about m3u8 urls. It refers to
the prefetching of file data inside the m3u8 stream manifest data, which
has nothing to do with mpv playlists when the m3u8 urls are opened as one
stream by ffmpeg.

The original comment was not very clear about this, so add more details
about this.

This reverts commit d4bf179e78.

DOCS/man/options.rst

@@ -4243,17 +4243,17 @@ Demuxer
 
 ``--prefetch-playlist=<yes|no>``
     Prefetch next playlist entry while playback of the current entry is ending
-    (default: no).
-
-    This does not prefill the cache with the video data of the next URL.
-    Prefetching video data is supported only for the current playlist entry,
-    and depends on the demuxer cache settings (on by default). This merely
-    opens the URL of the next playlist entry as soon the current URL is fully
-    read.
+    (default: no). This merely opens the URL of the next playlist entry as soon
+    as the current URL is fully read.
 
     This does **not** work with URLs resolved by the ``youtube-dl`` wrapper,
     and it won't.
 
+    This does not affect HLS streams (``.m3u8`` URLs). Such stream by itself is
+    internally a playlist of data segments, but is treated as a single media
+    item by mpv. HLS prefetching depends on the demuxer cache settings and is
+    on by default.
+
     This can occasionally make wrong prefetching decisions. For example, it
     can't predict whether you go backwards in the playlist, and assumes you
     won't edit the playlist.