Add warning about LE wildcard certificate for pages

What does this MR do?

We have clients confused custom domain certificates and wildcard certificates.

• For the first one with have integration with Let's Encrypt
• For the second we don't.

See comment and discussion.

This MR:

1. Remove confusing note about 2 IP's required for Let's Encrypt integration, since it's required for custom domains on it's own.
2. Add warning clarifying that current Let's Encrypt integration only covers custom domain and add link to omnibus issue for wildcard certificates for pages.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.
• 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