[auth-swift] Modernize Auth API docs

[paulb777]

Update Auth API docs implementation format so it generates correctly in Xcode documentation builds

• The PR primarily does the mechanical update and does not address missing documentation.
• It does fix obvious typos and a few obvious mistakes
• It only addresses the library and does not modernize test documentation
• Add back some docs that got missed in the Swift transition
• Split many API docs between summary and discussion with an extra blank line in the source

In general public API docs are mostly complete, while internal and private API documentation is hit or miss.

Note that the new doc format is more compact. Over 2000 lines deleted with no loss of info.

[google-oss-bot]

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[renkelvin]

Thanks Paul!

Out of curiosity, are these changes made by the style.sh script?

[paulb777]

@renkelvin The style script only fixes indentation and line lengths. Most of these changes were from ad-hoc editor scripts and manual review.

[paulb777]

Going to merge here. Let's address any follow up in subsequent PRs.