Documentation tweak around shared authorizers

[stuartsan]

What did you implement:

Documentation tweak to note that shared authorizers of type REQUEST are also fine

How did you implement it:

How can we verify it:

Per CloudFormation docs, which are referenced in the existing docs here:

The type of authorizer. Valid values include:
TOKEN: A custom authorizer that uses a Lambda function.
COGNITO_USER_POOLS: An authorizer that uses Amazon Cognito user pools.
REQUEST: An authorizer that uses a Lambda function using incoming request parameters.

And per #4325, it appears that REQUEST type authorizers are supported by serverless generally

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification config / commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: NO

[pmuens]

Thanks for the update @stuartsan 👍

LTGM