SERVER-6190 Clarify $week documentation.

Author: astaple

source/reference/aggregation.txt

@@ -844,8 +844,9 @@ argument and return a JavaScript "long" number.
    Takes a date and returns the week of the year as a number
    between 0 and 53.
 
-   Weeks start on Sundays and the days before the first Sunday of the
-   year are in "week 0."
+   Weeks begin on Sundays, and week 1 begins with the first Sunday
+   of the year. Days preceding the first Sunday of the year are in
+   week 0. This behavior is the same as strftime "%U".
 
 .. expression:: $year