Correct cluster labels documentation

[themantissa]

Description

Remove incorrect labels section and added example in cluster resource doc. Clarified in datasource doc.

Link to any related issue(s):

Type of change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• [X ] Documentation fix/enhancement

Required Checklist:

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

Further comments

[themantissa]

@PacoDw can you give this a look and see if it makes sense to you?

[PacoDw]

It seems nice, thanks 👍