[terraform-beta] docs for container_cluster and container_node_pool resources

[rrondeau]

Hi
This PR add the missing default_max_pods_per_node argument in the docs.
I also add the link to the official doc to make it clearer.
Thanks !

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[rrondeau]

I signed it!

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Hi! I'm the modular magician, I work on Magic Modules.
This PR seems not to have generated downstream PRs before, as of 8c6ee2f.

Pull request statuses

No diff detected in terraform-google-conversion.
No diff detected in Ansible.
No diff detected in Inspec.

New Pull Requests

I built this PR into one or more new PRs on other repositories, and when those are closed, this PR will also be merged and closed.
depends: hashicorp/terraform-provider-google-beta#563
depends: hashicorp/terraform-provider-google#3330

[rileykarson]

LGTM. Thanks @rrondeau!