Clarify "exists" and "only defined for" (#739)

* lack of hardware exposure omits things

* some things are only defined for a specific kind

* try to fix some rtcp DLSR things

* undefined in BWE things

* fix the remaining undefineds

* fix typo(s)

* Does not exist for other-kind

* omitted unless -> does not exist unless

* manually reflow

* Update webrtc-stats.html

---------

Co-authored-by: Varun Singh <vr000m@users.noreply.github.com>