[Threading docs] Clarify why a sequence is thread-safe.

As requested on the bug.

R=fdoray@chromium.org

Bug: 736531
Change-Id: I8f1fda7cc7e5973c72b2c9ff970c8a88620e0ca5
Reviewed-on: https://chromium-review.googlesource.com/c/1332768
Commit-Queue: François Doray <fdoray@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#607585}

docs/threading_and_tasks.md

@@ -190,8 +190,13 @@ base::SequencedTaskRunnerHandle::Get()->
 ## Using Sequences Instead of Locks
 
 Usage of locks is discouraged in Chrome. Sequences inherently provide
-thread-safety. Prefer classes that are always accessed from the same sequence to
-managing your own thread-safety with locks.
+thread-safety. Prefer classes that are always accessed from the same
+sequence to managing your own thread-safety with locks.
+
+**Thread-safe but not thread-affine; how so?** Tasks posted to the same sequence
+will run in sequential order. After a sequenced task completes, the next task
+may be picked up by a different worker thread, but that task is guaranteed to
+see any side-effects caused by the previous one(s) on its sequence.
 
 ```cpp
 class A {