HPCC-8576 Document Thor Memory settings

Signed-off-by: Jim DeFabia <jamesdefabia@lexisnexis.com>
Might be worth making a footnote something here, along the lines of : 
NB: Although a configuration may set combined upper memory limits that exceed total physical memory, Thor will not actually reserve the memory upfront and will only hit memory problems when and if your jobs use all of memory. So for example 2 Thor's that are configured to use all of memory could co-exist quite happily until a query on each are simultaneously using more memory than the OS has available.