doc(filter): support timezones other than UTC

Closes angular#10999

Author: shahata

src/ng/filter/filters.js

@@ -393,7 +393,9 @@ var DATE_FORMATS_SPLIT = /((?:[^yMdHhmsaZEw']+)|(?:'(?:[^']|'')*')|(?:E+|y+|M+|d
  *    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.
- * @param {string=} timezone Timezone to be used for formatting. Right now, only `'UTC'` is supported.
+ * @param {string=} timezone Timezone to be used for formatting. It understands UTC/GMT and the
+ *    continental US time zone abbreviations, but for general use, use a time zone offset, for
+ *    example, `'+0430'` (4 hours, 30 minutes east of the Greenwich meridian)
  *    If not specified, the timezone of the browser will be used.
  * @returns {string} Formatted string or the input if input is not recognized as date/millis.
  *