Documentation for saslauthd -> native ldap migration is missing

Please add documentation for migration from saslauthd to native ldap.

The process is the following:

1) Configure native ldap authentication ("enable LDAP authenticaion" should be turned on after completion of this step, "LDAP groups authorization" does not have to be enabled - if it's needed we recommend enabling it later, after migration is completed)
2) Disable authentication via saslauthd:
couchbase-cli setting-saslauthd -c 127.0.0.1:9001 -u Administrator -p password --enabled 0
3) Check that authentication for ldap users works (if not reenable saslauthd authenticaion).
4) Stop saslauthd daemon if not needed. At this point authentication for all external users is handled without saslauthd, so the daemon can be stopped if not needed for other purposes

Migration is complete!