Document where PlatformHandle validation happens in ipcz

ChannelMac says `incoming_handles_` should be validated, but where this
happens when using GetReadPlatformHandlesForIpcz() is not clear.

Change-Id: I8f1aeeb58fffdac15c86fb15ec82e53af238640b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4995345
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Robert Sesek <rsesek@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1217740}

mojo/core/channel_mac.cc

@@ -153,6 +153,10 @@ class ChannelMac : public Channel,
     return true;
   }
 
+  // Unlike GetReadPlatformHandles(), this does not validate the underlying
+  // PlatformHandle type here. Instead, ipcz does this in
+  // ipcz::Message::DeserializeFromTransport(), which is called by
+  // the AcceptParcel message deserializer (OnAcceptParcel).
   bool GetReadPlatformHandlesForIpcz(
       size_t num_handles,
       std::vector<PlatformHandle>& handles) override {