Bigtable: Minor doc update for Instance resource

[kevinsi4508]

Document the following:

• cluster can be removed on update and cause the resource to be managed on backend.
• Similar cluster is required doesn't work if the whole field is removed.
• ForceNew only the old and new clusters have the matching cluster ID.

b/255760356

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).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

[modular-magician]

Hello! I am a robot who works on Magic Modules PRs.

I've detected that you're a community contributor. @megan07, a repository maintainer, has been assigned to assist you and help review your changes.

❓ First time contributing? Click here for more details

---

Your assigned reviewer will help review your code by:

• Ensuring it's backwards compatible, covers common error cases, etc.
• Summarizing the change into a user-facing changelog note.
• Passes tests, either our "VCR" suite, a set of presubmit tests, or with manual test runs.

You can help make sure that review is quick by running local tests and ensuring they're passing in between each push you make to your PR's branch. Also, try to leave a comment with each push you make, as pushes generally don't generate emails.

If your reviewer doesn't get back to you within a week after your most recent change, please feel free to leave a comment on the issue asking them to take a look! In the absence of a dedicated review dashboard most maintainers manage their pending reviews through email, and those will sometimes get lost in their inbox.

---

[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 ( 2 files changed, 9 insertions(+), 6 deletions(-))
Terraform Beta: Diff ( 2 files changed, 9 insertions(+), 6 deletions(-))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2264
Passed tests 2023
Skipped tests: 241
Failed tests: 0

All tests passed in REPLAYING mode
View the build log

[melinath]

It looks like this is a followup related to hashicorp/terraform-provider-google#12884

[melinath]

lgtm