message: Added missing documentation for stddev
diff: @@ -698,6 +698,8 @@ The following standard SQL aggregation functions are supported. They may only be
  - `avg` returns the average of all values in the group. It works in
  double precision floating point only.
  +- `stddev` returns the standard deviation of all values in the group.
  + It works in double precision floating point only.
  - `sum` returns the sum of all values in the group. It works in
  double precision floating point only.
  - `min` returns the minimum of all values in the group.
  