Clarification of RtpPacket constructor in comment.

See also b/175210069 for more context.

Bug: None
Change-Id: I06e9848028c0f11362db373af54b42cbc67aee77
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/198780
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Henrik Grunell <henrikg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#32874}

modules/rtp_rtcp/source/rtp_packet.h

@@ -30,6 +30,8 @@ class RtpPacket {
   // packet creating and used if available in Parse function.
   // Adding and getting extensions will fail until |extensions| is
   // provided via constructor or IdentifyExtensions function.
+  // |*extensions| is only accessed during construction; the pointer is not
+  // stored.
   RtpPacket();
   explicit RtpPacket(const ExtensionManager* extensions);
   RtpPacket(const RtpPacket&);