aes-gcm internals: Clarify processing of last partial block, if any

[briansmith]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 91.17647% with 6 lines in your changes missing coverage. Please review.

Project coverage is 97.14%. Comparing base (a2a8e9b) to head (0dd3a35).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/aead/overlapping/partial_block.rs	85.71%	2 Missing and 2 partials ⚠️
src/aead/overlapping/base.rs	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2207      +/-   ##
==========================================
- Coverage   97.16%   97.14%   -0.03%     
==========================================
  Files         158      159       +1     
  Lines       20267    20298      +31     
  Branches      454      456       +2     
==========================================
+ Hits        19693    19719      +26     
- Misses        469      473       +4     
- Partials      105      106       +1     

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