docs(date): add missing doc about TZ behavior

Author: IgorMinar

src/ng/filter/filters.js

@@ -298,7 +298,8 @@ var DATE_FORMATS_SPLIT = /((?:[^yMdHhmsaZE']+)|(?:'(?:[^']|'')*')|(?:E+|y+|M+|d+
  *
  * @param {(Date|number|string)} date Date to format either as Date object, milliseconds (string or
  *    number) or various ISO 8601 datetime string formats (e.g. yyyy-MM-ddTHH:mm:ss.SSSZ and it's
- *    shorter versions like yyyy-MM-ddTHH:mmZ, yyyy-MM-dd or yyyyMMddTHHmmssZ).
+ *    shorter versions like yyyy-MM-ddTHH:mmZ, yyyy-MM-dd or yyyyMMddTHHmmssZ). If no timezone is
+ *    specified in the string input, the time is considered to be in the local timezone.
  * @param {string=} format Formatting rules (see Description). If not specified,
  *    `mediumDate` is used.
  * @returns {string} Formatted string or the input if input is not recognized as date/millis.