undo blocking on token polling ops; add docs & comments

[stevengum]

Fixes #4673 in main

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Copy-paste from PR #4675:

Specific Changes

• Change from Task.WaitAll() to Task.WhenAll() to not block background polling operations
  • This is spiritual a "reversion" of some code factoring in the 4.10.0 release
• Add comments/documentation explaining what the code is doing to prevent future breaks
• Fix minor typo

Testing

Manually tested these changes along with a fix for microsoft/BotFramework-WebChat#3439 and the workaround mentioned in #4674 with Direct Line ASE and Direct Line Speech. The OAuth flow works as expected.

Note: No magic-code OAuth flows are not supported outside of regular Direct Line with Enhanced Authentication.