Change recommended operation IDL syntax

[mtdowling]

The old operation syntax is now deprecated in favor of a syntax that's
more consistent with the rest of the IDL.

The following operation,

operation Foo(Input) -> Output errors[X, Y, Z]

Is now defined as:

operation Foo {
    input: Input,
    output: Output,
    errors: [X, Y, Z]
}

Why?

• This new syntax is more consistent with other shapes like resources,
  operations, structures, etc. This consistency makes it easier to
  implement parsers, formatters, etc.
• Virtually all actual models define operations with errors. When an
  operation uses long input names, output, and defines errors, they
  either are one really long line or span multiple lines. How an
  operation definition was broken across multiple lines is something
  that has been approached with great inconsistency within AWS and
  Amazon. Using a syntax format that removes all ambiguity will help
  drive consistency in models, thereby making them easier to read.

The old syntax is still supported, but no longer documented. A
validation event is emitted when it is encountered. Support for the old
syntax will be removed in Smithy 1.0.

Issue #, if available:

Description of changes:

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