Add note re: subgroups not executing group webhooks

What does this MR do?

This MR adds a note to clarify that group level webhooks are not triggered by projects in subgroups.

A customer asked about this functionality, and I was actually a bit confused by it myself since it was not documented and it's a bit unintuitive. So I hope this addition will clarify that this is expected behavior and not a bug.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team