docs: clarify resolver scope — when source/destination DSNs differ

"Cross-instance migrations" was ambiguous: it read like cloud-to-cloud
or dedicated-DB migration. The actual scope is narrower — any case
where the source project's stored DSN doesn't apply to the destination
project (cross-host within the same instance, cross-region, mixed
shared-tables migration). Reword the property, constructor, and
inline write-site comments to say that explicitly.

Author: abnegate

src/Migration/Destinations/Appwrite.php

@@ -104,11 +104,13 @@ class Appwrite extends Destination
     protected $getDatabasesDB;
 
     /**
-     * Resolves the DSN written into the destination's `_databases.database` for
-     * a migrated database. When unset, the attribute is left blank and the
-     * runtime falls back to the destination project's DSN — correct for legacy
-     * single-DSN projects, but cross-instance / multi-type setups must inject
-     * a resolver so source DSNs don't bleed into the destination metadata.
+     * Resolves the DSN written into the destination's `_databases.database`
+     * for a migrated database. When the source and destination projects don't
+     * share the same DSN — e.g. one project is on a host the other isn't —
+     * pass a resolver so the destination metadata carries its own DSN instead
+     * of the source's. When unset, the attribute is left blank and the
+     * runtime falls back to the destination project's DSN at read time, which
+     * is safe for single-host single-type setups.
      *
      * @var (callable(Database $resource): string)|null
      */
@@ -150,7 +152,7 @@ class Appwrite extends Destination
      * @param callable(UtopiaDocument $database):UtopiaDatabase $getDatabasesDB
      * @param array<array<string, mixed>> $collectionStructure
      * @param OnDuplicate $onDuplicate Behavior when a row with an existing $id is encountered.
-     * @param (callable(Database $resource): string)|null $getDatabaseDSN Resolver for the destination's `_databases.database` value. Required for cross-instance migrations to prevent the source DSN from being written into the destination project's metadata.
+     * @param (callable(Database $resource): string)|null $getDatabaseDSN Resolver for the destination's `_databases.database` value. Pass when the destination project's DSN differs from the source's, so the destination row carries its own DSN instead of inheriting the source's.
      */
     public function __construct(
         string $project,
@@ -185,8 +187,9 @@ public function __construct(
     /**
      * Resolve the DSN written into the destination's `_databases.database`.
      * Without a resolver, leave it blank — the source DSN must never be
-     * propagated, since cross-instance source/destination DSNs differ and
-     * propagation routes destination reads to the wrong host (see PR #151).
+     * propagated as the default, since when source and destination DSNs
+     * differ propagation routes destination reads to the wrong host (the
+     * regression PR #151 introduced).
      */
     private function resolveDestinationDsn(Database $resource): string
     {
@@ -569,7 +572,7 @@ protected function createDatabase(Database $resource): bool
             '$updatedAt' => $updatedAt,
             'originalId' => empty($resource->getOriginalId()) ? null : $resource->getOriginalId(),
             'type' => empty($resource->getType()) ? 'legacy' : $resource->getType(),
-            // Source and destination can be in different locations; never write the source DSN here.
+            // Resolved by the destination's resolver (or left blank); never copy the source's DSN by default.
             'database' => $this->resolveDestinationDsn($resource),
         ]));