doc: another pass on authentication components

[coryan]

I made a pass trying to assume less about what the reader may know about authentication. Motivated by the incorrect magic for numbered items in Doxygen.

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This change is 

[google-cloud-cpp-bot]

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[codecov]

Codecov Report

Base: 93.84% // Head: 93.84% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (c10cd5d) compared to base (427350f).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #10300   +/-   ##
=======================================
  Coverage   93.84%   93.84%           
=======================================
  Files        1592     1592           
  Lines      144868   144868           
=======================================
+ Hits       135956   135957    +1     
+ Misses       8912     8911    -1     

Impacted Files	Coverage Δ
google/cloud/pubsub/subscriber_connection_test.cc	98.78% <0.00%> (-1.22%)	⬇️
...cloud/pubsub/internal/subscription_session_test.cc	98.49% <0.00%> (-0.17%)	⬇️
...torage/examples/storage_service_account_samples.cc	97.64% <0.00%> (+0.58%)	⬆️
google/cloud/internal/async_connection_ready.cc	93.61% <0.00%> (+4.25%)	⬆️

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[scotthart]

Reviewed all commit messages.
Reviewable status: 0 of 1 files reviewed, 1 unresolved discussion (waiting on @coryan)

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google/cloud/doc/guac.dox line 39 at r1 (raw file):

As we mentioned in passing, authentication supports both user accounts and
service accounts. These are referred as "principals". User accounts represent
a developers, administrator, or any other person who interacts with GCP

s/^a// or s/developers/developer

Code quote:

a developers

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