Clarify documentation for self-destructing callbacks.

It's also okay to copy a RepeatingCallback onto the stack before running
it if running it may cause it to be deleted.

Bug: 1161804
Change-Id: I4ee5d81b5ab82505c4c3b02f872d3400d3d93ceb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2607886
Auto-Submit: Greg Thompson <grt@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Greg Thompson <grt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#840012}

docs/callback.md

@@ -372,9 +372,9 @@ void DoSomething(const base::RepeatingCallback<double(double)>& callback) {
 
 If running a callback could result in its own destruction (e.g., if the callback
 recipient deletes the object the callback is a member of), the callback should
-be moved before it can be safely invoked. (Note that this is only an issue for
-RepeatingCallbacks, because a OnceCallback always has to be moved for
-execution.)
+be moved or copied onto the stack before it can be safely invoked. (Note that
+this is only an issue for RepeatingCallbacks, because a OnceCallback always has
+to be moved for execution.)
 
 ```cpp
 void Foo::RunCallback() {