Update ldap#security section

What does this MR do?

This MR updates Security section of ldap.md mentioning that LDAP users are expected to have unique email addresses. Otherwise account take over is possible, as already stated but a paragraph above newly added one.

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