message: Update some important change log notes with links
diff: a `merge_rows` instruction applied and then rolled back. This could have
  caused corruption if this scenario was triggered but since sync does not use
  the `merge_rows` instruction in this way, this is a preventative fix.
  - PR [#2503](https://github.com/realm/realm-core/pull/2503)
  + PR [#2503](https://github.com/realm/realm-core/pull/2503).
  * Fixed an assertion on a corner case of reallocation on large arrays.
  PR [#2500](https://github.com/realm/realm-core/pull/2500).
  Fixes issue [#2451](https://github.com/realm/realm-core/issues/2451).
  ### Breaking changes
  * Disable copying of various classes to prevent incorrect use at compile time.
  + PR [#2468](https://github.com/realm/realm-core/pull/2468).
  * History type enumeration value `Replication::hist_Sync` renamed to
  - `Replication::hist_SyncClient`.
  + `Replication::hist_SyncClient`. PR [#2482](https://github.com/realm/realm-core/pull/2482).
  * Bumps file format version from 6 to 7 due to addition of a 10th element into
  `Group::m_top`. The new element is the history schema version, which is
  crucial for managing the schema upgrade process of sync-type histories in a
  way that is independent of core's Realm file format. The bump is necessary due
  to lack of forwards compatibility. The changes are backwards compatible, and
  - automatic upgrade is implemented.
  + automatic upgrade is implemented. PR [#2481](https://github.com/realm/realm-core/pull/2481).
  * New pure virtual methods `get_history_schema_version()`,
  `is_upgradable_history_schema()`, and `upgrade_history_schema()` in
  - `Replication` interface.
  + `Replication` interface. PR [#2481](https://github.com/realm/realm-core/pull/2481).
  ### Enhancements
  * Support setting Mixed(Timestamp) through the transaction logs.
  + PR [#2507](https://github.com/realm/realm-core/pull/2507).
  * Implement comparison of Mixed objects containing Timestamp types.
  + PR [#2507](https://github.com/realm/realm-core/pull/2507).
  * Allow query for size of strings, binaries, linklists and subtables:
  Query q = table->where().size_equal(2, 5);
  Query q = table1->column<SubTable>(2).size() == 5;
  + PR [#2504](https://github.com/realm/realm-core/pull/2504).
  * New history type enumeration value `Replication::hist_SyncServer`. This allows
  for the sync server to start using the same kind of in-Realm history scheme as
  - is currently used by clients.
  + is currently used by clients. PR [2482](https://github.com/realm/realm-core/pull/2482).
  -----------
  * `AppendBuffer` gained support for move construction/assignment, and had its
  growth factor reduced to 1.5.
  * Methods on the `Replication` interface were made virtual to allow override.
  + PR [#2462](https://github.com/realm/realm-core/pull/2462).
  * The order of emission for some instructions in the transaction log was changed
  with respect to carrying out the effect of the instruction on the database, to
  allow implementors of the `Replication` interface a semi-consistent view of
  