doc: describe native turnserver scope more clearly
doc: describe native turnserver scope more clearly

backport of https://github.com/webrtc/webrtc-org/pull/236

BUG=none

Change-Id: I03ba8ef6ad0c778a2b44978e4a19c2aabad4b001
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/172581
Reviewed-by: Harald Alvestrand <ce816cd73d80f958d82de1df356058c2d2bca378@webrtc.org>
Commit-Queue: Harald Alvestrand <ce816cd73d80f958d82de1df356058c2d2bca378@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#30967}

ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc