DOCS: document boosting is not supported by the elasticsearch backend

[jochengcd]

This was removed when adding the elasticsearch2 backend #1247

Before the docs was also not quite correct #1368 for elasticsearch, but that is moot now. That issue can be closed.

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[jochengcd]

This was removed when adding the elasticsearch2 backend #1247

Before the docs was also not quite correct for elasticsearch, but that is moot now. #1368 That issue can be closed.