docs: make clearer that retry until/when does not support backoff

[manofthepeace]

Happened as discussed here; #1427

[cescoffier]

Thanks!

[codecov]

Codecov Report

Merging #1439 (fb2f908) into main (d883a75) will increase coverage by 0.14%.
Report is 1 commits behind head on main.
The diff coverage is n/a.

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #1439      +/-   ##
============================================
+ Coverage     89.26%   89.40%   +0.14%     
- Complexity     3359     3367       +8     
============================================
  Files           459      459              
  Lines         13454    13454              
  Branches       1655     1655              
============================================
+ Hits          12010    12029      +19     
+ Misses          799      792       -7     
+ Partials        645      633      -12     

Files	Coverage Δ
.../main/java/io/smallrye/mutiny/groups/UniRetry.java	90.38% <ø> (ø)

... and 12 files with indirect coverage changes

[jponge]

Thanks @manofthepeace

Can you please sign your commit? This is required to merge. https://docs.github.com/en/authentication/managing-commit-signature-verification/about-commit-signature-verification