[8.19] [ES|QL] Specify population in StdDev docs (#129225) (#129231)

There are 2 types of Standard Deviation: population and
sample, this commit clarifies that the existing is population.

Author: limotova

docs/reference/esql/functions/description/std_dev.asciidoc

@@ -36,12 +36,12 @@ public class StdDev extends AggregateFunction implements ToAggregator {
 
     @FunctionInfo(
         returnType = "double",
-        description = "The standard deviation of a numeric field.",
+        description = "The population standard deviation of a numeric field.",
         isAggregation = true,
         examples = {
             @Example(file = "stats", tag = "stdev"),
             @Example(
-                description = "The expression can use inline functions. For example, to calculate the standard "
+                description = "The expression can use inline functions. For example, to calculate the population standard "
                     + "deviation of each employee's maximum salary changes, first use `MV_MAX` on each row, "
                     + "and then use `STD_DEV` on the result",
                 file = "stats",