Add a comment to explain how errno is set

Since it is nontrivial which modifies `errno` is modified in
ExecuteForTask(), add a comment that explains this.

Bug: 213617178
Test: Compile-tested only.
Change-Id: I49ce9c8054fdc59e61b2e5f9ffe6f16743a94401
Signed-off-by: Bart Van Assche <bvanassche@google.com>