docs(fetch): multiple fields with the same name in java and .net (#30105)

Reference https://github.com/microsoft/playwright/issues/28070

docs/src/api/class-formdata.md

@@ -14,6 +14,72 @@ FormData form = FormData.create()
 page.request().post("http://localhost/submit", RequestOptions.create().setForm(form));
 ```
 
+## method: FormData.add
+* since: v1.43
+- returns: <[FormData]>
+
+Adds a field to the form. File values can be passed either as `Path` or as `FilePayload`.
+Multiple fields with the same name can be added.
+
+```java
+import com.microsoft.playwright.options.FormData;
+...
+FormData form = FormData.create()
+    // Only name and value are set.
+    .add("firstName", "John")
+    // Name and value are set, filename and Content-Type are inferred from the file path.
+    .add("attachment", Paths.get("pic.jpg"))
+    // Name, value, filename and Content-Type are set.
+    .add("attachment", new FilePayload("table.csv", "text/csv", Files.readAllBytes(Paths.get("my-tble.csv"))));
+page.request().post("http://localhost/submit", RequestOptions.create().setForm(form));
+```
+
+```csharp
+var multipart = Context.APIRequest.CreateFormData();
+// Only name and value are set.
+multipart.Add("firstName", "John");
+// Name, value, filename and Content-Type are set.
+multipart.Add("attachment", new FilePayload()
+{
+    Name = "pic.jpg",
+    MimeType = "image/jpeg",
+    Buffer = File.ReadAllBytes("john.jpg")
+});
+// Name, value, filename and Content-Type are set.
+multipart.Add("attachment", new FilePayload()
+{
+    Name = "table.csv",
+    MimeType = "text/csv",
+    Buffer = File.ReadAllBytes("my-tble.csv")
+});
+await Page.APIRequest.PostAsync("https://localhost/submit", new() { Multipart = multipart });
+```
+
+### param: FormData.add.name
+* since: v1.43
+- `name` <[string]>
+
+Field name.
+
+### param: FormData.add.value
+* since: v1.43
+- `value` <[string]|[boolean]|[int]|[Path]|[Object]>
+  - `name` <[string]> File name
+  - `mimeType` <[string]> File type
+  - `buffer` <[Buffer]> File content
+
+Field value.
+
+### param: FormData.add.value
+* since: v1.43
+* langs: csharp
+- `value` <[string]|[boolean]|[int]|[Object]>
+  - `name` <[string]> File name
+  - `mimeType` <[string]> File type
+  - `buffer` <[Buffer]> File content
+
+Field value.
+
 ## method: FormData.create
 * since: v1.18
 * langs: java
@@ -36,7 +102,7 @@ FormData form = FormData.create()
     // Name and value are set, filename and Content-Type are inferred from the file path.
     .set("profilePicture1", Paths.get("john.jpg"))
     // Name, value, filename and Content-Type are set.
-    .set("profilePicture2", new FilePayload("john.jpg", "image/jpeg", Files.readAllBytes(Paths.get("john.jpg"))));
+    .set("profilePicture2", new FilePayload("john.jpg", "image/jpeg", Files.readAllBytes(Paths.get("john.jpg"))))
     .set("age", 30);
 page.request().post("http://localhost/submit", RequestOptions.create().setForm(form));
 ```
@@ -52,6 +118,7 @@ multipart.Set("profilePicture", new FilePayload()
     MimeType = "image/jpeg",
     Buffer = File.ReadAllBytes("john.jpg")
 });
+multipart.Set("age", 30);
 await Page.APIRequest.PostAsync("https://localhost/submit", new() { Multipart = multipart });
 ```