Document using custom geodiff working dir on server

[varmar05]

[tomasMizera]

Is this going to help people somehow? Can't we just have a reasonable default as was till now?

[varmar05]

Is this going to help people somehow? Can't we just have a reasonable default as was till now?

The default is reasonable, and in that case users do not have to do anything. The problem is when they change it to something else - we have to sum up consequences :-)