manpage: clarify writes to playlist-pos

I don't know what should happen when the same value is written to the
property. It seems that it would be more natural if it were ignored
(since that's also what is done with options now), but you could argue
the other way around as well. In any case, changing it silently could
leads to user scripts etc. breaking, so don't change it now.

Instead, add blabla to the manpage to put the responsibility on the
user, so when we suddenly change it one day, we can blame any breakages
on someone else.

See: #7501

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