Update memory guidance to reflect default `pipeline.buffer.type` in Logstash `9.x`

[robbavey]

The beats input includes a section on memory usage, which talks about the use of "off-heap" and direct memory in the plugin, and provides appropriate guidance.

Logstash 9.x changes the default Netty memory allocation to heap from direct memory, which changes this guidance - allocations are now performed on heap, unless the setting has been explicitly changed.

We should update the documentation to reflect this.