Update docs for Swift interceptors

[rebello95]

Updates the documentation to reflect the changes in https://github.com/connectrpc/connect-swift/pull/186.

The copy matches Interceptor.swift's inline documentation from that PR with the addition of an auth example here.

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Name	Status	Preview	Comments	Updated (UTC)
connect	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Oct 18, 2023 10:19pm

[rebello95]

@eseay can you please take a look at this when you get a chance?

[akshayjshah]

LGTM to me, but @eseay's opinion will probably be more helpful :)

[rebello95]

Thanks for the reviews/feedback @akshayjshah @eseay!