Decoding of fields from the change index must be implemented for each
index type (Lucene and ElasticSearch). In the past it happened that
decoding was only implemented for one of the index types and
implementing it for the other index type was forgotten. Decrease the
chance of this happening by adding a comment in each change index
implementation to remind that decoding fields has to be done for all
index types.
Change-Id: I1a7e024c4b4330fb4cba69a6a243879e284d5b86
Signed-off-by: Edwin Kempin <ekempin@google.com>