Document how to handle xtdb/crux out-of-memory errors

[praseodym]

In bigger installations, xtdb/crux might need more memory to function properly. This will show up as xtdb repeatedly crashing with Terminating due to java.lang.OutOfMemoryError: Java heap space in its log.

We should document that this should be handled by giving xtdb more memory. The xtdb-http-multinode README contains some short instructions for this, namely increasing the Xmx (max heap) and maybe also MaxDirectMemorySize in the JAVA_TOOL_OPTIONS environment variable for the crux Docker container. The default for this variable is -Xms128M -Xmx512M -XX:MaxDirectMemorySize=512M -XX:+ExitOnOutOfMemoryError.

In the Debian package there are two different variables, namely MAX_MEMORY and MAX_DIRECT_MEMORY (see xtdb-http-multinode.service). I think these should be set using a systemd unit file override (systemctl edit).