[Threading docs] Cross-reference FAQ and main docs

and follow-up on nits to https://chromium-review.googlesource.com/c/chromium/src/+/1261855

R=fdoray@chromium.org

Change-Id: I2ca5305af71fa89e13be10e40779641362529a42
Reviewed-on: https://chromium-review.googlesource.com/c/1342770
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610416}