docs(ngMock/$timeout): recommend `$verifyNoPendingTasks()` when appropriate

For historical reasons, `$timeout.verifyNoPendingTasks()` throws if
there is any type of task pending (even if it is not a timeout). When
calling `$timeout.verifyNoPendingTasks()`, the user is most likely
interested in verifying pending timeouts only, which is now possible
with `$verifyNoPendingTasks('$timeout')`.

To raise awareness of `$verifyNoPendingTasks()`, it is mentioned in the
error message thrown by `$timeoutverifyNoPendingTasks()` if none of the
pending tasks is a timeout.