fix(functions): clarify auth comments

[ace-n]

Cloud Run stuff was showing up in the Functions docs; this should fix that.

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 4 region tags.

• samples/idtokens-serverless.js:39, tag cloudrun_service_to_service_auth
• samples/idtokens-serverless.js:47, tag functions_bearer_token
• samples/idtokens-serverless.js:57, tag functions_bearer_token
• samples/idtokens-serverless.js:58, tag cloudrun_service_to_service_auth

You are about to delete 2 region tags.

• samples/idtokens-serverless.js:41, tag functions_bearer_token
• samples/idtokens-serverless.js:44, tag cloudrun_service_to_service_auth

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