aio: clarify self.ops difference in fallback

In io_uring, we have to keep track of the pending operations that have
not yet been completed, but in Fallback that's not the case.

src/aio/Fallback.zig

@@ -324,6 +324,7 @@ fn start(self: *@This(), id: u16) !void {
 }
 
 fn submit(self: *@This()) !bool {
+    // unlike io_uring, this is not only submitted ops, but also pending ops
     if (self.ops.empty()) return false;
     defer self.prev_id = null;
     self.pfd.add(.{ .fd = self.source.fd, .events = std.posix.POLL.IN, .revents = 0 }) catch unreachable;