Add some information about the memory footprint of keeping a TypeDB database open

[alexjpwalker]

We have observed that the TypeDB Java process consumes around 80MB of memory for each database that exists.

This is because it keeps all of its databases open at all times, to optimise I/O performance.

Information should be added to our docs that mentions this.

[izmalk]

Is it still relevant and accurate?

[alexjpwalker]

Yes, the above remains true today.