fix: Add documentation generation for operations.

[sichanyoo]

This PR adds in-line documentation generation for operations.

Issue #

awslabs/aws-sdk-swift#1033

Description of changes

Issue #1033 is resolved by adding documentation generation for errors thrown by operations.
When a service client protocol and service client are generated, for each and every operation, the code generator now generates in-line documentation for parameters, return type, and each and every error thrown by that operation (both errors common to that service, and errors bound to that operation), in the standard Swift in-line documentation format accepted by Swift DocC, like the following :

    /// This operation is cool.
    ///
    /// - Parameter TestStreamOpInput : [no documentation found]
    ///
    /// - Returns: `TestStreamOpOutputResponse` : [no documentation found]
    ///
    /// - Throws: One of the exceptions listed below __Possible Exceptions__.
    ///
    /// __Possible Exceptions:__
    /// - `SomeError` : You don't have permission.

Error name is extracted from name of the error structs from the corresponding operation shape.
Error cause is extracted from documentation on the error struct from the the corresponding service model file. If no documentation is provided for a given error struct in the original model file, "[no documentation found]" is appended as the error cause.

Below are example code-generated output, as well as what it looks like in Swift DocC:

Generated in-line documentation:

Quick-help view of in-line doc:

Documentation built with generated in-line doc:

This change was tested using tests in the smithy-swift project, as well as codegen tests in aws-sdk-swift project.

Scope

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[jbelkins]

A couple things:

• We should change the format to the following:

    /// - Throws: `BucketAlreadyExists`: The requested bucket name is not available.
    ///
    /// `BucketAlreadyOwnedByYou`: The bucket you tried to create already exists, and you own it.

Note the backticks around the exception name and the empty comment line between exceptions. This renders as follows in Xcode inline docs:

• We should add Parameters and Returns to the inline function comment docs as well, for completeness. We can write a separate ticket for that feature though.

[dayaffe]

Just a few more Kotlin syntax suggestions!

[dayaffe]

One more change to make use of Kotlin syntax and then I am good to approve!

[dayaffe]

LGTM