backend: raise error for infinite register clash during class definition

[superlopuh]

It feels weird to raise an error only on the first time that a register from the infinite set is greated, this PR changes the mechanism to instead raise during class definition.

Also adds a bit of documentation for this.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.26%. Comparing base (2ec6429) to head (e5e3cd6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5661   +/-   ##
=======================================
  Coverage   86.26%   86.26%           
=======================================
  Files         403      403           
  Lines       56881    56875    -6     
  Branches     6565     6564    -1     
=======================================
- Hits        49068    49065    -3     
+ Misses       6282     6280    -2     
+ Partials     1531     1530    -1     

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