Document breaking changes for completion and context suggester

[areek]

breaking changes for #14410

[clintongormley]

I'd replace this para with:

The completion suggester has undergone a complete rewrite. This means that the
syntax and data structure for fields of type `completion` have changed, as
have the syntax and response of completion suggester requests.

For indices created before Elasticsearch 5.0.0, `completion` fields and the
completion suggester will continue to work as they did in Elasticsearch 2.x.
However, it is not possible to run a completion suggester query across indices
created in 2.x and indices created in 5.x.

It is strongly recommended to reindex indices containing 2.x `completion`
fields in 5.x to take advantage of the new features listed below.

NOTE: You will need to change the structure of the completion field values
when reindexing.

[clintongormley]

Small suggestions, other than that LGTM. Thanks for the good writeup

[areek]

Thanks @clintongormley for the review!