Document default_join_distribution_mode query option

Based on new query option from IMPALA-5381.

Default setting is:

	DEFAULT_JOIN_DISTRIBUTION_MODE: [0]

which I take (also from looking at the gerrit) to mean that 0 = BROADCAST, 1 = SHUFFLE.

I presume that it is strictly speaking a numeric argument that could be extended to more values in future, as opposed to a Boolean argument that is forever restricted to 0 or 1.