Document InnerHandler null requirement for DI

[bbrandt]

What kind of change does this PR introduce?
docs update

What is the current behavior?
See #1403

What is the new behavior?
Clearly documented

What might this PR break?
documentation only. Helps devs get started more easily with Auth in Refit.

Please check if the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

• Addressing [BUG] The 'InnerHandler' property must be null. #1403

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (dc07cfb) 90.08% compared to head (93c530f) 90.08%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1569   +/-   ##
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  Coverage   90.08%   90.08%           
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  Files          33       33           
  Lines        1685     1685           
=======================================
  Hits         1518     1518           
  Misses        167      167           

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[github-actions]

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