CLOUDP-423175: Add IPA-117-description override for Principal.onBehalfOf

## Proposed changes

The dev OpenAPI Spec release failed because the `xgen-IPA-117-description`
Spectral rule flagged `#/components/schemas/Principal/properties/onBehalfOf`
for a missing description.

Add an override in `tools/spectral/ipa/ipa-spectral.yaml` to turn off the
`xgen-IPA-117-description` rule for that component, following the existing
pattern for fields that can't be documented.

_Jira ticket:_ CLOUDP-423175

Closes #1372

## Checklist

- [ ] I have signed the [MongoDB CLA](https://www.mongodb.com/legal/contributor-agreement)
- [ ] I have added tests that prove my fix is effective or that my feature works

### Changes to Spectral
- [x] I have read the [README](../tools/spectral/README.md) file for Spectral Updates

Author: mongodb-sage-bot[bot]

tools/spectral/ipa/ipa-spectral.yaml

@@ -33,6 +33,7 @@ overrides:
   - files:
       - '**#/components/schemas/ClusterProviderSettings/properties/providerName' # dynamic field which can't be documented
       - '**#/components/schemas/DataLakeStoreSettings/properties/provider'
+      - '**#/components/schemas/Principal/properties/onBehalfOf'
     rules:
       xgen-IPA-117-description: 'off'
   - files: