Add warning about using pocl_jll on top of script

[blegat]

This is a bit obvious from the perspective of a C library that is non interactive but the behavior may seem counter-intuitive from the Julia perspective.
I've checked through the OpenCL-ICD-Loader and there does not seem to be a way to force it to refresh the list so it seems opening a new Julia session is the right recommendation

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.96%. Comparing base (d154022) to head (eddf69d).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #295      +/-   ##
==========================================
- Coverage   73.12%   72.96%   -0.17%     
==========================================
  Files          12       12              
  Lines         614      614              
==========================================
- Hits          449      448       -1     
- Misses        165      166       +1     

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[maleadt]

LGMT, thanks. It's a pity that the ICD works as statically as it does, I'd love to have an API to register new back-ends instead of the env var shenanigans we have to do now. Alas.