Document cache quota 2GB headroom restriction

The following discussion from !563 (merged) should be addressed:

• @Qinusty commented on a discussion: (+2 comments)

  Agreed, and we probably want to make sure this headroom is documented somewhere as it may confuse people trying to set their cache_quota based on their available system storage. E.g. You may be told that you don't have enough space because you have specified within 2GB of your system capacity.

Description

There is currently no mention within the documentation of this 2GB headroom, this could cause some confusion to users who are trying to set their quota to their available space.