Merge branch 'stable-3.1'

* stable-3.1:
  Bump Bazel version to 2.2.0
  Bump required Bazel version to 2.1.0
  Upgrade bazlets to latest stable-2.16 to build with 2.16.16 API
  Documentation: Remove bazel issue
  Documentation: Refactor to match other plugin documentation
  Documentation: Add missing alternative for plugin in-tree testing
  Upgrade bazlets to latest stable-2.16
  Upgrade bazlets to latest stable-2.16
  Upgrade bazlets to latest stable-2.15
  Upgrade bazlets to latest stable-2.14

Change-Id: I0f0e7e4f918b4d7f0fa1af2024a050bdea658a67