Expand on use of separate name attributes in LDAP

What does this MR do?

The name attribute in LDAP fetches the user's visible name in profiles but often the ideal value for this is split between two different fields that are configurable via the use of first_name and last_name.

The docs were not very clear on how to force use of the latter (split) configuration over name, doable by intentionally setting it to something non-existent.

This change adds minor notes to first_name and last_name config descriptions, and adds an example to name that shows how to force the use of the split name sources.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability 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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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