Document configuring temp. directories used for file uploads

[landreev]

This is a child issue of a huge "refactor upload" issue #6656.
This is something we've been meaning to do literally for years; really a shame we haven't done it yet.

This part of the system is somewhat counter-intuitive (total of 3 different temp. locations involved) and it keeps confusing installation admins. But documenting it should be fairly trivial.
One slightly non-trivial thing we should investigate in the process is whether there is a simpler/more elegant way to configure the upload directory used by PrimeFaces than the one that's currently available. (see the parent issue for more details)

[poikilotherm]

Hi @landreev are you aware of my #8983 sitting around?

[landreev]

I was about to mention that this issue may have been created as a result of some confusion/miscommunication on this end (I was told that this part of #6656 needed to be addressed, as a new thing. They must have meant your PR). I am aware of yours now; will close this.