manpage: --demuxer-seekable-cache is useless

De-emphasize it, since a user should usually not use it. This _could_ be
used to make the cache seekable with --cache=no, but it's better and
more intuitive to use --cache=yes. As such, the only use of this is for
debugging. I'm not quite sure if this should be removed entirely, but I
still see some value in it (for example if you want the cache lookahead,
but you're using a stream where cache seeking is somehow broken).

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