message: Add avgInArray, minInArray, maxInArray functions into docs.
diff: @@ -557,6 +557,9 @@ Here is the list of built-in functions:
  | sum(par1, par2, ...) | Returns the summary of passed parameters. |
  | avg(par1, par2, ...) | Returns the average value for passed parameters. |
  | sumInArray({questionName}, 'propertyName') | Returns the summary for array of objects {questionName} by property 'propertyName'. `sumInArray('matrixdynamic', 'total') > 1000</` |
  +| avgInArray({questionName}, 'propertyName') | Returns the average value for array of objects {questionName} by property 'propertyName'. `avgInArray('matrixdynamic', 'quantity') > 4</` |
  +| minInArray({questionName}, 'propertyName') | Returns the minimum value for array of objects {questionName} by property 'propertyName'. `minInArray('matrixdynamic', 'quantity') > 1</` |
  +| maxInArray({questionName}, 'propertyName') | Returns the maximum value for array of objects {questionName} by property 'propertyName'. `maxInArray('matrixdynamic', 'quantity') > 10</` |
  If you feel there is a need in a particular function, then [write us](https://github.com/surveyjs/surveyjs/issues) about it.
  