Cosmos_DisableMetadata_Append.json: Clarify description

#### Details of the scenario you tried and the problem that is occurring

Per microsoftdocs/azure-docs#94920

This policy's description isn't clear about what's actually happening when you implement the policy. The description doesn't make clear what **key-based metadata write access** really is. We also can't update the docs on this since it's autogenerated.

#### Verbose logs showing the problem

N/A

#### Suggested solution to the issue

Update description with this text:

```
This policy enables you to ensure all Azure Cosmos DB accounts disable write access for clients connecting using any account keys.
```

#### If policy is Guest Configuration - details about target node

N/A

Locate Cosmos_DisableMetadata_Append.json in the policy definitions and read its current description. Replace the description with the suggested wording, then verify that the file contains the exact text and that the policy definition remains valid JSON.

Written by the indexing model from the issue text.