HPCC-10339 Document SKEW&THRESHOLD for TABLE&BUILD

Signed-off-by: Jim DeFabia <jamesdefabia@lexisnexis.com>
As noted in the related doc. issue (HPCC-10340), the default skew is 1.0 / \<number of nodes\>. 
See: https://github.com/hpcc-systems/HPCC-Platform/pull/6015#