Include note about 30 day grace period in email verification docs

What does this MR do?

This change affects documentation where we mention that requiring email verification will prevent newly registered users from logging in. Our current language presents this as a security feature, while changes were made to allow access for 30 days to enhance user experience.

I think it's important to clarify this in case customers are relying on immediate email verification as part of their security strategy. Marking this as a fix since the existing docs do not accurately describe the current behavior of the feature.

Screenshots

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