Add PermissionsBoundary to the list of supported globals in the docs.

[jasonmk]

*Issue #661

Description of changes:
Documentation fix

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

@@           Coverage Diff            @@
##           develop     #791   +/-   ##
========================================
  Coverage    94.23%   94.23%           
========================================
  Files           67       67           
  Lines         2688     2688           
  Branches       478      478           
========================================
  Hits          2533     2533           
  Misses          80       80           
  Partials        75       75

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ce8d889...8f84374. Read the comment docs.