docs: Add some details about API Gateway resources

Author: Radek Simko

website/source/docs/providers/aws/r/api_gateway_integration.html.markdown

@@ -48,7 +48,11 @@ The following arguments are supported:
 * `http_method` - (Required) The HTTP method (`GET`, `POST`, `PUT`, `DELETE`, `HEAD`, `OPTION`)
 * `type` - (Required) The integration input's type (HTTP, MOCK, AWS)
 * `uri` - (Optional) The input's URI (HTTP, AWS). **Required** if `type` is `HTTP` or `AWS`.
+  For HTTP integrations, the URI must be a fully formed, encoded HTTP(S) URL according to the RFC-3986 specification . For AWS integrations, the URI should be of the form `arn:aws:apigateway:{region}:{subdomain.service|service}:{path|action}/{service_api}`. `region`, `subdomain` and `service` are used to determine the right endpoint.
+  e.g. `arn:aws:apigateway:eu-west-1:lambda:path/2015-03-31/functions/arn:aws:lambda:eu-west-1:012345678901:function:my-func/invocations`
 * `credentials` - (Optional) The credentials required for the integration. For `AWS` integrations, 2 options are available. To specify an IAM Role for Amazon API Gateway to assume, use the role's ARN. To require that the caller's identity be passed through from the request, specify the string `arn:aws:iam::\*:user/\*`.
 * `integration_http_method` - (Optional) The integration HTTP method
   (`GET`, `POST`, `PUT`, `DELETE`, `HEAD`, `OPTION`).
+  Not all methods are compatible with all `AWS` integrations.
+  e.g. Lambda function [can only be invoked](https://github.com/awslabs/aws-apigateway-importer/issues/9#issuecomment-129651005) via `POST`.
 * `request_templates` - (Optional) A map of the integration's request templates.

website/source/docs/providers/aws/r/api_gateway_method.html.markdown

@@ -42,3 +42,5 @@ The following arguments are supported:
 * `authorization` - (Required) The type of authorization used for the method
 * `api_key_required` - (Optional) Specify if the method requires an API key
 * `request_models` - (Optional) A map of the API models used for the request's content type
+  where key is the content type (e.g. `application/json`)
+  and value is either `Error`, `Empty` (built-in models) or `aws_api_gateway_model`'s `name`.