Add javadoc comment for PeerConnection.dispose.

Specifically calling out issue 3721 ("dispose can't be called from a
callback"), which developers frequently run into.

BUG=webrtc:3721
NOTRY=True

Review-Url: https://codereview.webrtc.org/3013573002
Cr-Original-Commit-Position: refs/heads/master@{#19804}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 43697f6da56c0c6eca8967a4fef57beb589d8990
1 file changed