docs: add comments about auth token requirements

[jullanggit]

Adds comments to the documentation for distributed compilation explaining the requirement of the token being a valid HTTP header value.
Hopefully this saves the next person running into this a bit of time :)

[sylvestre]

thanks

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.04%. Comparing base (9f6cba4) to head (df698fb).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2583      +/-   ##
==========================================
+ Coverage   72.03%   72.04%   +0.01%     
==========================================
  Files          66       66              
  Lines       36953    36953              
==========================================
+ Hits        26619    26623       +4     
+ Misses      10334    10330       -4     

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