docs(numberFilter): fix explanation of default fraction size

The default fraction size for the number filter is actually computed
from the `NUMBER_FORMATS.PATTERNS.maxFrac` value in the current locale.

Closes angular#3157

Author: petebacondarwin

src/ng/filter/filters.js

@@ -62,7 +62,9 @@ function currencyFilter($locale) {
  * If the input is not a number an empty string is returned.
  *
  * @param {number|string} number Number to format.
- * @param {(number|string)=} [fractionSize=2] Number of decimal places to round the number to.
+ * @param {(number|string)=} fractionSize Number of decimal places to round the number to.
+ * If this is not provided then the fraction size is computed from the current locale's number
+ * formatting pattern. In the case of the default locale, it will be 3.
  * @returns {string} Number rounded to decimalPlaces and places a “,” after each third digit.
  *
  * @example