Clarify use of Matrix room.

[crertel]

Neither the README nor the Matrix channel info mentions that the Matrix is only for urgent issues:

The matrix is supposed to be used only for urgent issues:

This PR updates the public policy documentation to match the position of the mod team.

[dcarosone]

We had been hoping that the need for this was temporary and the flooding of horror would be resolved sooner. That seems less likely to be soon, the longer it goes on. The use of the room (and the setting of alerts) has had to evolve in response; we still hope it can shift again in the other direction, where we can turn of alerts and rely on mentions again. I've just updated the room name, and will review the rest of the wording too.

[crertel]

As mentioned in Matrix, it should probably be sufficient to just inline the relevant bit from the Discourse:

Both the current README and the channel header (unless you click-through) are about what one can do--if we want to keep people from doing something else, it should be explicit about the cannot. Also, if it's something that spams mods in off-hours, that'd be helpful to mention too.

[dcarosone]

Have also updated the pinned message in the room:

NOTICE: The moderation bot is currently set to remove all posted images, due to abuse. For the same reason, we have had to restrict the use of this room to urgent reports. Please take discussion elsewhere, perhaps to NixOS Matrix Discussion for the general state of matrix problem-solving. We apologise for the inconvenience.