Docs: clarify calculation of sigma and lambda in function_score (#20267)

- Using log() to indicate natural log can add some confusion when trying to further adjust/tweak scores. Other parts of the API (field_value_factor on this same page) use 'ln' and 'log', so this change should be more consistent
- Fixes #20027
- I generated the images using http://latex2png.com/ at a resolution of 150 which seemed to be about the same size as before

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

@@ -359,6 +359,7 @@ 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`
 
+// \sigma^2 = -scale^2/(2 \cdot ln(decay))
 image:images/sigma_calc.png[]
 
 See <<gauss-decay>> for graphs demonstrating the curve generated by the `gauss` function.
@@ -374,6 +375,7 @@ 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`
 
+// \lambda = ln(decay)/scale
 image:images/lambda_calc.png[]
 
 See <<exp-decay>> for graphs demonstrating the curve generated by the `exp` function.