Update guards docs to new signature of Guard and createGuard

[codecov[bot]]

Codecov Report

Merging #13 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #13   +/-   ##
=======================================
  Coverage   87.08%   87.08%           
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  Files          60       60           
  Lines        1084     1084           
  Branches      197      197           
=======================================
  Hits          944      944           
  Misses        139      139           
  Partials        1        1

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[pie6k]

Thanks!