docs(transactions): expand docs on relations and slices (#1200)

docs/pages/docs/walkthrough/transactions.md

@@ -27,7 +27,7 @@ model Comment {
 }
 ```
 
-## Successful scenario
+## Example
 
 A simple transaction could look as follows. Just omit the `Exec(ctx)`, and provide the Prisma calls
 to `client.Prisma.Transaction`:
@@ -53,6 +53,59 @@ log.Printf("first post result: %+v", firstPost.Result())
 log.Printf("second post result: %+v", secondPost.Result())
 ```
 
+## Multiple transactions
+
+If you have many transaction items or need to construct a transaction dynamically, you can use the `PrismaTransaction` type:
+
+```go
+// create two posts at once and run in a transaction
+var txns []db.PrismaTransaction
+
+for _, title := range []string{"First Post", "Second Post"} {
+  txn := client.Post.CreateOne(
+    db.Post.Published.Set(true),
+    db.Post.Title.Set(title),
+  ).Tx()
+  txns = append(txns, txn)
+}
+
+if err := client.Prisma.Transaction(txns...).Exec(ctx); err != nil {
+  panic(err)
+}
+```
+
+## Setting relations with foreign keys
+
+When setting relations with foreign keys, you have to set the IDs of the related records. For example, to create a post with multiple comments, you would do the following:
+
+```go
+postID := "12345"
+
+txns := []db.PrismaTransaction{
+  client.Post.CreateOne(
+    db.Post.Published.Set(true),
+    db.Post.Title.Set("First Post"),
+    db.Post.ID.Set(postID),
+  ).Tx(),
+  client.Comment.CreateOne(
+    db.Comment.Content.Set("First comment"),
+    db.Comment.Post.Link(
+      db.Post.ID.Equals(postID),
+    ),
+  ).Tx(),
+  client.Comment.CreateOne(
+    db.Comment.Content.Set("Second comment"),
+    db.Comment.Post.Link(
+      db.Post.ID.Equals(postID),
+    ),
+  ).Tx(),
+}
+
+if err := client.Prisma.Transaction(txns...).Exec(ctx); err != nil {
+  panic(err)
+}
+```
+
 ## Failure scenario
 
 Let's say we have one post record in the database: