docs: add Auth to supported Api Global properties

[keetonian]

Issue #, if available:
N/A
Description of changes:
Add Auth to API globals docs. It is supported in the code: https://github.com/awslabs/serverless-application-model/blob/master/samtranslator/plugins/globals/globals.py#L42
Description of how you validated changes:

Checklist:

• Write/update tests
• make pr passes
• Update documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-io]

Codecov Report

Merging #899 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #899   +/-   ##
========================================
  Coverage    94.65%   94.65%           
========================================
  Files           69       69           
  Lines         3011     3011           
  Branches       559      559           
========================================
  Hits          2850     2850           
  Misses          85       85           
  Partials        76       76

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