data_source_aws_security_group: add description

[darkowlzz]

From #1940 I came across https://www.terraform.io/docs/providers/aws/d/security_group.html#description where description is an attribute reference. But in real, description isn't available. This change adds description to data_source_aws_security_group.

This doesn't f-ixes #1940, which talks about description for the SG rules, which I see already exist 🤷‍♀️.

[Ninir]

Hey @darkowlzz

This looks good! 👍

Just left a nitpick. Also, can you ad the attribute in the documentation? would help people knows this exists.

Thanks!

[Ninir]

Optional is used when the user can pass a value for this attribute. Since we are only retrieving the description (and we don't need it to retrieve the Security Group info), this is not needed here and can be removed

[darkowlzz]

@Ninir thanks for the review.

Removed Optional and combined with the same commit.

About adding the attribute to documentation, isn't it already there in the docs? https://github.com/terraform-providers/terraform-provider-aws/blob/master/website/docs/d/security_group.html.markdown#attributes-reference

Should I add it to some other docs too?

[Ninir]

LGTM, thanks a lot @darkowlzz! 👍

$ make testacc TEST=./aws TESTARGS='-run=TestAccDataSourceAwsSecurityGroup'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./aws -v -run=TestAccDataSourceAwsSecurityGroup -timeout 120m
=== RUN   TestAccDataSourceAwsSecurityGroup
--- PASS: TestAccDataSourceAwsSecurityGroup (22.36s)
PASS
ok  	github.com/terraform-providers/terraform-provider-aws/aws	22.391s

[ghost]

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