[Cronet] Add safety notes about adding / removing and destroying listeners.

API users should know that adding / removing listeners may not take
effect for already-stared requests -- add a note.

Also, some of the earlier comments on listener and executor lifetime
turned out to be wrong -- fix these, and simplify the wording.

Bug: 879208
Change-Id: Ibd4e07993b598b7d7aafb416d9e571227c1db3ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1556379
Reviewed-by: Misha Efimov <mef@chromium.org>
Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Commit-Queue: Caleb Raitto <caraitto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652250}
1 file changed