prefer-date-now: Elaborate on rule rationale (#946)

Author: jdanil

docs/rules/prefer-date-now.md

@@ -1,6 +1,6 @@
 # Prefer `Date.now()` to get the number of milliseconds since the Unix Epoch
 
-[`Date.now()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/now) is shorter and nicer than [`new Date().getTime()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/getTime).
+[`Date.now()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/now) is shorter and nicer than [`new Date().getTime()`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/getTime), and avoids unnecessary instantiation of `Date` objects.
 
 This rule is fixable.