Adding comments about threading around CreatePeerConnectionFactory.

This has confused a lot of developers (understandably).

R=pthatcher@webrtc.org

Review URL: https://codereview.webrtc.org/1828463002 .

Cr-Original-Commit-Position: refs/heads/master@{#12105}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a8415fe9ea381946148498f1fe7fd0b2b04f02ec
2 files changed