Add samesite documentation to cookies.rst

[ruckc]

Document samesite in cookies.

[codecov]

Codecov Report

Merging #2016 (37a00ee) into master (ffca01f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2016   +/-   ##
=======================================
  Coverage   91.45%   91.45%           
=======================================
  Files          28       28           
  Lines        3171     3171           
  Branches      571      571           
=======================================
  Hits         2900     2900           
  Misses        192      192           
  Partials       79       79           

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[ahopkins]

These docs are being deprecated. RTD will only be sphinx auto doc (see #2006).

This PR should be make on this repo instead: https://github.com/sanic-org/sanic-guide