added documentation of isoweek (#6237)

Co-authored-by: nitish jha <nitishjha@nitishs-MacBook-Air.local>

man/IDateTime.Rd

@@ -175,6 +175,13 @@ They can round or truncate to hours and minutes.
 Note for ITime's with 30 seconds, rounding is inconsistent due to rounding off a 5.
 See 'Details' in \code{\link{round}} for more information.
 
+Functions like \code{week()} and \code{isoweek()} provide week numbering functionality.
+\code{week()} computes completed or fractional weeks within the year,
+while \code{isoweek()} calculates week numbers according to ISO 8601 standards,
+which specify that the first week of the year is the one containing the first Thursday.
+This convention ensures that week boundaries align consistently with year boundaries,
+accounting for both year transitions and varying day counts per week.
+
 }
 
 \value{