Document and expose WAN-oriented replication backlog sizing guidance

## Problem

The replication backlog defaults to a five-second age target and 0.5% of `maxmemory`. Under
continuing writes, a WAN outage longer than this window can evict the required LSN and force a full
database synchronization.

The behavior is intentional, but the defaults target short connection restoration rather than
rack or DC partitions.

This behavior is documented, but its effect on WAN recovery still deserves a dedicated issue.

## References

https://github.com/dragonflydb/dragonfly/blob/980c70331d194d7133079917668fec269444c65c/src/server/journal/journal_slice.cc#L23-L49

https://github.com/dragonflydb/dragonfly/blob/980c70331d194d7133079917668fec269444c65c/src/server/journal/journal_slice.cc#L176-L207

https://github.com/dragonflydb/dragonfly/blob/980c70331d194d7133079917668fec269444c65c/src/server/dflycmd.cc#L486-L495

https://github.com/dragonflydb/dragonfly/issues/7994

Read the replication backlog defaults in src/server/journal/journal_slice.cc at the referenced sections, then inspect the related handling in src/server/dflycmd.cc and issue #7994. Document how the five-second and maxmemory-based defaults affect WAN recovery, and expose clear sizing guidance for that scenario.

Written by the indexing model from the issue text.