[webrtc-stats] Pull Request: Added note about channels for all sample based metrics

henbos has just submitted a new pull request for https://github.com/w3c/webrtc-stats:

== Added note about channels for all sample based metrics ==
Fixes #448.

Alternatively we could avoid copy-pasting the same paragraph all over the places by defining this once in the beginning of the document, perhaps that is preferred, what do you think?

See https://github.com/w3c/webrtc-stats/pull/501