Update documentation of ymd describing reliance on strptime

  Reported in #289

R/parse.r

@@ -15,9 +15,12 @@
 ##' truncated formats. For example \code{ymd} with \code{truncated = 2} will also
 ##' parse incomplete dates like \code{2012-06} and \code{2012}.
 ##'
-##' NOTE: \code{ymd} family of functions are based on \code{\link{strptime}}
-##' which currently correctly parses "\%y" format, but fails to parse "\%y-\%m"
-##' formats.
+##' NOTE: \code{ymd} family of functions are based on `parse_date_time` and thus
+##' directly drop to internal C parser for numeric months, but use R's
+##' `strptime` for alphabetic months. This implies that some of the `strptime`'s
+##' limitations are inherited by lubridate's parser. For example truncated
+##' formats (like %Y-%b) will not be parsed. Numeric truncated formats (like
+##' %Y-%m) are handled correctly by lubridate's C parser.
 ##'
 ##' As of version 1.3.0, lubridate's parse functions no longer return a
 ##' message that displays which format they used to parse their input. You can