docs: explain scanning array_agg (and other PG arrays) into Go slices

Users hitting a query that uses `array_agg` (or any PG array-returning
expression) and who try to scan its result into a struct field often see
"unable to assign to *[]int64" and assume the feature is missing. The
two-step recipe from #1900 — scan the array scalar into a slice with
`QueryRow` / slice-typed `Scan`, and compose slice fields into structs
via `CollectRows` / `RowToStructByName` — is the canonical answer but
isn't currently discoverable from the package docs.

Add a short paragraph with both forms to the PostgreSQL Data Types
section of `doc.go` so `go doc pgx` / pkg.go.dev surface the recipe.

Closes #1900

Signed-off-by: Ali <alliasgher123@gmail.com>

Author: alliasgher

doc.go

@@ -74,6 +74,27 @@ pgx uses the [pgtype] package to converting Go values to and from PostgreSQL val
 directly and is customizable and extendable. User defined data types such as enums, domains,  and composite types may
 require type registration. See that package's documentation for details.
 
+PostgreSQL arrays (including results from set-returning aggregates such as array_agg)
+can be scanned directly into a matching Go slice. For scalar columns, pass the slice
+as the scan destination:
+
+    var ids []int64
+    err := conn.QueryRow(ctx, "select array_agg(id) from things").Scan(&ids)
+
+For a column that is part of a row, combine the slice with the usual row-to-struct
+helpers. A struct field of slice type will pick up the array_agg column when
+collected via [CollectRows] and [RowToStructByName] (or
+[RowToAddrOfStructByPos]):
+
+    type ThingEntry struct {
+        GroupID  int64
+        ThingIDs []int64 `db:"thing_ids"`
+    }
+
+    rows, _ := conn.Query(ctx,
+        "select group_id, array_agg(thing_id) as thing_ids from things group by group_id")
+    entries, err := pgx.CollectRows(rows, pgx.RowToStructByName[ThingEntry])
+
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