manpage: update --playlist comments

Using the --playlist option is no longer recommended.

A while ago, mpv rewrote all playlist parsers and added some minimal
security mechanisms (like not allowing local file access or unsafe
protocols in remote playlists). Further, mpv can load playlists by
passing them as normal file arguments, without the option.

Now, --playlist is needed only in these situations:
1) loading plaintext files
2) disabling additional security mechanisms
   (e.g. using a remote playlist to play local files)

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