update comments for key argument

[imsteev]

_get_keys(key) in jws.py allows for the key argument to already be in dictionary format. This PR updates comments to clarify for important functions.

[codecov-io]

Codecov Report

Merging #103 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #103   +/-   ##
=======================================
  Coverage   93.58%   93.58%           
=======================================
  Files          13       13           
  Lines         998      998           
=======================================
  Hits          934      934           
  Misses         64       64

Impacted Files	Coverage Δ
jose/jwt.py	94.19% <ø> (ø)	⬆️
jose/jws.py	94.11% <ø> (ø)	⬆️

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[mpdavis]

lgtm