DOCSP-34393 added median to the agg options table (#635)

* added median to the agg options table

Author: kyuan-mongodb

source/encoding-channels/aggregation-options.txt

@@ -63,6 +63,10 @@ Numbers
      - Averages all values in the field.
      - :manual:`$avg </reference/operator/aggregation/avg/>`
 
+   * - ``median``
+     - Approximates the median value in the field.
+     - :manual:`$median </reference/operator/aggregation/median/>`
+
    * - ``min``
      - Returns the minimum value in the field.
      - :manual:`$min </reference/operator/aggregation/min/>`