Document breaking changes introduced with #13752

[javanna]

With #13752 we introduced quite some breaking changes to the java api. We should review what we did and properly document those breaking changes in the migration guide for 3.0. Also, we should consider deprecating in 2.x some the methods that we have removed from SearchRequest and SearchRequestBuilder etc.

[javanna]

Also #14384 and #14270 made the same changes to ExplainRequest, ExplainRequestBuilder, ValidateQuery and ValidateQueryRequestBuilder. These should be documented the same way as we do it for the search request.

[clintongormley]

@javanna / @colings86 has this been done yet?

[colings86]

Not yet no. There are still some small changes being made to the search request so I was waiting for it to settle more.