Add documentation to note "-gb" Prestissimo Configs are using GiB units and not gB units

Author: minhancao

presto-docs/src/main/sphinx/presto_cpp/properties.rst

@@ -12,6 +12,10 @@ For information on catalog configuration properties, see :doc:`Connectors </conn
 
 For information on Presto C++ session properties, see :doc:`properties-session`.
 
+NOTE: While some of the configuration properties below with "-gb" in their names 
+show gigabytes (gB; 1 gB equals 1000000000 B), it is actually 
+gibibytes (GiB; 1 GiB equals 1073741824 B).
+
 .. contents::
     :local:
     :backlinks: none