[PIG] Document work-around for Pig 1 reducer problem

Since in many cases Pig internal assumptions force the use of 1 reducer,
no matter the job properties or the number of splits, document the
settings that disable this behavior (and which unfortunately cannot be
set directly by es-hadoop).

fix #294

(cherry picked from commit 3f7aee4574d881ef83aa657a4fc72ae100cc4a4f)