Hint user to disable caching if enabling `print_ir`

[v01dXYZ]

It's stupid but the documentation about print_ir could remind the user to disable caching with offline_cache=False. Otherwise, there won't be any IR printed if the kernel was already cached.

[hughperkins]

Yes, I get caught by this all the time. Feel free to submit a PR.

• bonus points for detecting when both QD_DUMP_IR=1, and qd offline cache has not been disabled, and printing out something usefl, I feel.
  (really, I've wasted hours multiple times because of this 😅)