Make tpgtools generate beta docs into GA as well for parity with mmv1.

[modular-magician]

Also, minor changes to make sure that the docs are a little bit better - allow overrides of documentation values, for instance.

Also, includes the actual description of the resource, which, at the moment is not acceptable in the DCL. Will need to implement an override in the DCL.

Fixes the worst of hashicorp/terraform-provider-google#10252, bringing the urgency down to "we need better docstrings for these fields". I'll get to that soon.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
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all: fixed an issue where some documentation for new resources was not showing up in the GA provider if it was beta-only.

Derived from GoogleCloudPlatform/magic-modules#5325