Add warnings to similarities that may return 0 as a score.

[jpountz]

Some similarities may return scores that are equal to zero in some conditions. We should probably add warnings to the docs about such similarities as this is usually impractical in practice since having matches on terms that contribute 0 to the score doesn't make such documents rank higher as is usually expected.

• AxiomaticF3Log
• AxiomaticF3Exp
• DFISimilarity
• LMDirichletSimilarity

[elasticmachine]

Pinging @elastic/es-search-aggs

[jpountz]

Actually the axiomatic similarities are not exposed.