Document new velocity system properties

[Warriorrrr]

Documents PaperMC/Velocity#1743

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[WouterGritter]

See PaperMC/Velocity#1743 (comment)

Though this change in Velocity aligns it with vanilla MC (~32kb plugin message payload limit), it seems like modded minecraft increases this limit. I think it would be useful to mention this here in the docs; increase this value (velocity.max-plugin-message-payload-size) when you're getting errors and are using server-side mods on the backend servers.

Example error log:

[11:46:06 ERROR]: [server connection] ... -> ...: exception encountered in com.velocitypowered.proxy.connection.backend.BackendPlaySessionHandler@775d12ab
io.netty.handler.codec.CorruptedFrameException: Packet sent for class com.velocitypowered.proxy.protocol.packet.PluginMessagePacket was too big (expected 229378 bytes, got 515269 bytes)