DOCS/edl-mpv: document a dumb thing

EDLs can be provided either as external file, or "inline" as a big
edl:// URL. There is no difference between them, except if it's loaded
from an external file, there is some weird filename sanitation going on
(see fix_filenames() in demux_edl.c). It seems this is intended to be a
security mechanism, but probably makes no sense at all.

Note that playlists are allowed to access anything locally. One
difference to playlists is that the EDL code lacks the "security"
mechanism when accessing playlist entries (see handling of the
playlist_entry.stream_flags field - EDL would need something similar),
so don't remove that, as I'm unaware of the exact consequences.

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