Fix: Clarify usage of schema in Table

[nevzheng]

Our users highlighted unclear documentation, which I have clarified in this PR.

Fixes: hashicorp/terraform-provider-google#15377

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• 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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Release Note Template for Downstream PRs (will be copied)

bigquery: Clarify usage of `schema` in `google_bigquery_table`

[modular-magician]

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 1 file changed, 18 insertions(+), 2 deletions(-))
Terraform Beta: Diff ( 1 file changed, 18 insertions(+), 2 deletions(-))

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 1 file changed, 18 insertions(+), 2 deletions(-))
Terraform Beta: Diff ( 1 file changed, 18 insertions(+), 2 deletions(-))

[melinath]

Please also adjust the surrounding lines - unfortunately GitHub does not support multi-line suggestions.

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 1 file changed, 17 insertions(+), 7 deletions(-))
Terraform Beta: Diff ( 1 file changed, 17 insertions(+), 7 deletions(-))