identity: improve the documentation for Principal.Name()

[nsmith5]

Summary

Clarifies that Name must match the proof of possession.

@haydentherapper let me know if you can thing of a better name for the method. Happy to add that in here. Thought I'd at least improve the docs while I remembered to.

[codecov-commenter]

Codecov Report

Merging #579 (3b7c790) into main (8d307b5) will decrease coverage by 0.67%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #579      +/-   ##
==========================================
- Coverage   44.29%   43.62%   -0.68%     
==========================================
  Files          21       21              
  Lines        1517     1513       -4     
==========================================
- Hits          672      660      -12     
- Misses        771      781      +10     
+ Partials       74       72       -2     

Impacted Files	Coverage Δ
pkg/ca/x509ca/common.go	0.00% <0.00%> (-32.61%)	⬇️
pkg/ca/googleca/v1/googleca.go	52.25% <0.00%> (ø)
pkg/challenges/challenges.go	29.71% <0.00%> (+0.93%)	⬆️

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[haydentherapper]

Thanks for the added documentation. I think this is good for now - Naming is hard :)

Ideally I'd like to call this Subject, but it's not accurate in terms of OIDC.