message: [gbc] Clean up service AST documentation
  * Remove duplicate "Service" section.
  * Remove incorrect "Service method" that talked about passing messages
  and services, which was never implemented in gbc.
diff: @@ -628,27 +628,6 @@ A JSON object representing a forward declaration has the following properties:
  ]
  }
  -### Service
  -
  -A JSON object representing a `Service` declaration has the following properties:
  -
  - {
  - "tag": "Service",
  - "declNamespaces": [
  - ],
  - "declName": "ServiceName",
  - "declParams": [
  - ],
  - "declAttributes": [
  - ],
  - "serviceMethods": [
  - ]
  - }
  -
  -where:
  -
  -- `serviceMethods` is an array of zero or more [methods](#service-method).
  -
  Qualified name
  --------------
  @@ -837,78 +816,6 @@ Other complex types are:
  arguments for a generic user defined type. The property is optional and
  may be omitted for non-generic types.
  -Service method
  ---------------
  -
  -A JSON object representing a `Method` has the following properties:
  -
  - {
  - "tag": "Tag",
  - "methodName": "MethodName",
  - "methodAttributes": [
  - ],
  - "methodResult": {
  - },
  - "methodInput": {
  - }
  - }
  -
  -where
  -
  -- `tag` is one of the following string values:
  - - `"Sink"` to represent a one-way method that doesn't return any result.
  - - `"Function"` to represent a method that returns a result.
  -- `methodName` is a string.
  -- `methodAttributes` is an array of zero or more [attributes](#attribute).
  -- `methodResult` is an object representing [message](#message) returned from
  -the method as the result.
  -- `methodInput` is an object representing [message](#message) accepted by the
  -method as the input.
  -
  -### Message
  -
  -A JSON object representing a `Message` has the following properties:
  -
  - {
  - "messagePayload": {
  - },
  - "messageService": {
  - }
  - }
  -
  -where
  -
  -- `messagePayload` is `null` or an object representing [type](#type) of data
  -payload carried by the message. The type must be a user defined struct.
  -- `messageService` is `null` or an object representing [type of a service](#service-type)
  -that can be passed in the message, e.g. as a callback.
  -
  -### Service type
  -
  -Service type can be specified using one of the following JSON objects:
  -
  - {
  - "typeParam": {
  - }
  - }
  -
  -or
  -
  - {
  - "declaration": {
  - },
  - "arguments": {
  - }
  - }
  -
  -where
  -
  -- `typeParam` is an object representing [type parameter](#type-parameter).
  -- `declaration` is a [service declaration](#service).
  -- `arguments` is an array of zero or more [types](#type) representing type
  -arguments for a generic service. The property is optional and may be omitted
  -for non-generic services.
  -
  Runtime Schema
  ==============
  