update documentation

Amplitude/Amplitude.h

@@ -239,7 +239,7 @@
  @param eventType                The name of the event you wish to track.
  @param eventProperties          You can attach additional data to any event by passing a NSDictionary object with property: value pairs.
  @param groups                   You can specify event-level groups for this user by passing a NSDictionary object with groupType: groupName pairs. Note the keys need to be strings, and the values can either be strings or an array of strings.
- @param timestamp                You can specify a custom timestamp by passing an NSNumber representing the milliseconds since epoch UTC time. If nil is passed in, then the event will be timestamped with the current time.
+ @param timestamp                You can specify a custom timestamp by passing an NSNumber representing the milliseconds since epoch UTC time. We recommend using [NSNumber numberWithLongLong:milliseconds] to create the value. If nil is passed in, then the event will be timestamped with the current time.
  @param outOfSession             If YES, will track the event as out of session. Useful for push notification events.
 
  @see [Tracking Events](https://github.com/amplitude/amplitude-ios#tracking-events)

documentation/html/Classes/Amplitude.html

@@ -1463,7 +1463,7 @@ <h4 class="method-subtitle parameter-title">Parameters</h4>
 
 					<tr>
 						<th scope="row" class="argument-name"><code>timestamp</code></th>
-						<td><p>You can specify a custom timestamp by passing an NSNumber representing the milliseconds since epoch UTC time. If nil is passed in, then the event will be timestamped with the current time.</p></td>
+						<td><p>You can specify a custom timestamp by passing an NSNumber representing the milliseconds since epoch UTC time. We recommend using [NSNumber numberWithLongLong:milliseconds] to create the value. If nil is passed in, then the event will be timestamped with the current time.</p></td>
 					</tr>
 
 					<tr>