Documentation: Add aws_api_gateway_request_validator to documentation

[jerthom]

The aws_api_gateway_request_validator resource was added in hashicorp/terraform-provider-aws#1064 but it isn't listed in the docs as a provider, nor as an argument for aws_api_gateway_method.

[lologhi]

I've started doing it here.

[bflad]

Hi folks 👋 Thanks for reporting this here. We typically track resource specific documentation requests in the provider repositories themselves as that is where the documentation pages live. In this case, I submitted a pull request for the missing documentation here: hashicorp/terraform-provider-aws#5110