Improve jwt.decode key doc

[erny]

PR for #96

[erny]

Perhaps we should add some tests here?

[codecov]

Codecov Report

Merging #198 (03dd4ce) into master (a2a4c31) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #198   +/-   ##
=======================================
  Coverage   93.51%   93.51%           
=======================================
  Files          16       16           
  Lines        1728     1728           
=======================================
  Hits         1616     1616           
  Misses        112      112           

Impacted Files	Coverage Δ
jose/jwt.py	100.00% <ø> (ø)

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