Add details around index=no

Should resolve this issue:
https://github.com/elasticsearch/elasticsearch.github.com/issues/430

Appears there was a breaking change in 0.18 that was not clearly
documented:
https://github.com/elasticsearch/elasticsearch/issues/1087?source=cc

Clarify that index=no defaults to include_in_all to false. Also fix the
statement around why you may want
to set index=no (storing in source, store=yes, include_in_all=enabled).