doc: improve cluster doc to highlight that provider_volume_type=STANDARD is not available for NVMW clusters

[andreaangiolillo]

Description

This was discovered as part of a query in our internal hashicorp slack channel
Link to any related issue(s):

Type of change:

• Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR.
• Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR.
• This change requires a documentation update
• Documentation fix/enhancement

Required Checklist:

• I have signed the MongoDB CLA
• I have read the contribution guidelines
• I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• I have added any necessary documentation (if appropriate)
• I have run make fmt and formatted my code

Further comments

[wtrocki]

Users will still get error with a similar message when they still try to use it?

[andreaangiolillo]

yes, there is a validation in the endpoint that will return an error. I created this ticket https://jira.mongodb.org/browse/CLOUDP-197623 to improve the open API doc of the endpoint as well

[wtrocki]

Awesome