Add doc on rank_feature(s) negative score impact

Add a warning about consequences of negative score impact
for documents that don't have values for rank_feature(s)
fields.

Related to elastic#69994

docs/reference/mapping/types/rank-feature.asciidoc

@@ -58,3 +58,10 @@ the <<query-dsl-rank-feature-query,`rank_feature`>> query to modify the scoring
 in such a way that the score decreases with the value of the feature instead of
 increasing. For instance in web search, the url length is a commonly used
 feature which correlates negatively with scores.
+
+With `positive_score_impact` set to `false` for a `rank_feature` field,
+we recommend that every document that participates in a query
+has a value for this field. Otherwise, a document that has the negative feature
+would contribute to a higher score than a document that doesn't have the feature,
+which is counter-intuitive as we want documents with negative features
+to be ranked lower.

docs/reference/mapping/types/rank-features.asciidoc

@@ -94,3 +94,9 @@ the <<query-dsl-rank-feature-query,`rank_feature`>> query to modify the scoring
 in such a way that the score decreases with the value of the feature instead of
 increasing.
 
+With `positive_score_impact` set to `false` for a `rank_features` field,
+we recommend that every document that participates in a query
+has a value for this field. Otherwise, a document that has the negative feature
+would contribute to a higher score than a document that doesn't have the feature,
+which is counter-intuitive as we want documents with negative features
+to be ranked lower.