Document service action and Application Default Credentials

[lacostej]

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[lacostej]

I signed the CLA!

[googlebot]

CLAs look good, thanks!

[bentiffany]

I second @Etherdaemon's comment–please double check that you're referencing the right class. Thank you, @lacostej, for attempting to add this documentation. If I'd seen this initially, I would have saved quite a few hours of work.

[hxiong388]

@lacostej Thanks for submitting the PR. I have incorporated this change into #110.