[doc] Correct decay function equations in function_score description

Impact of decay and scale was missing from the equations.

Closes #6983

docs/reference/query-dsl/queries/function-score-query.asciidoc

@@ -273,18 +273,33 @@ Normal decay, computed as:
 +
 image:images/Gaussian.png[]
 
+where image:images/sigma.png[] is computed to assure that the score takes the value `decay` at distance `scale` from `origin`+-`offset`
+
+image:images/sigma_calc.png[]
+
+[horizontal]
 `exp`::
 
 Exponential decay, computed as:
 +
 image:images/Exponential.png[]
 
+where again the parameter image:images/lambda.png[] is computed to assure that the score takes the value `decay` at distance `scale` from `origin`+-`offset`
 
+image:images/lambda_calc.png[]
+
+[horizontal]
 `linear`::
+
 Linear decay, computed as:
 +
 image:images/Linear.png[].
-+
+
+
+where again the parameter `s` is computed to assure that the score takes the value `decay` at distance `scale` from `origin`+-`offset`
+
+image:images/s_calc.png[]
+
 In contrast to the normal and exponential decay, this function actually
 sets the score to 0 if the field value exceeds twice the user given
 scale value.