Document expected thread safety of the TaskQueue interface
Document expected thread safety of the TaskQueue interface

Bug: webrtc:12182
Change-Id: I06fc2e2fca98cb9e03d664d5bd8143ae06184314
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/200702
Reviewed-by: Niels Moller <0057859e47d61d90529d0f66d9ef692cefedb46a@webrtc.org>
Reviewed-by: Harald Alvestrand <ce816cd73d80f958d82de1df356058c2d2bca378@webrtc.org>
Commit-Queue: Danil Chapovalov <55de8d9c2b5977da2dd48f967f21f9d606b172a5@webrtc.org>
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#33032}

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