Clarify in documentation that ATLASSIAN-SECURITY is only necessary to import passwords from other Atlassian products

The encryption method description says:

For compatibility between Atlassian products you must use ATLASSIAN-SECURITY.

This is an oversimplification: it's only necessary if you plan to migrate a user database between Crowd and other Atlassian products, and not necessary for general use of a Crowd server.

Fix the documentation to make this clear.