Small fixup to recent agent changes

[hynek]

There was a small race condition between @glyph and me. This PR should fix that. :)

• it links the Agent docs which should be useful if someone is reading about it
• show a more practical example instead of an opaque function
• simplify the agent logic. kwargs isn’t used further down so no need to remove agent from it
• add a doc string to the test
• some whitespace fixes

[codecov-io]

Current coverage is 95.84%

Merging #108 into master will not affect coverage as of b426a85

@@            master    #108   diff @@
======================================
  Files           20      20       
  Stmts         1417    1417       
  Branches       113     113       
  Methods          0       0       
======================================
  Hit           1358    1358       
  Partial         19      19       
  Missed          40      40       

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[glyph]

Anything tested using mock.Mock makes me nervous and I wish we had more real testing 😞 . But that's a different PR.