docs: add note about large file multipart uploads (#2025)

Co-authored-by: David Pedersen <david.pdrsn@gmail.com>

axum/src/extract/multipart.rs

@@ -50,6 +50,13 @@ use std::{
 /// let app = Router::new().route("/upload", post(upload));
 /// # let _: Router = app;
 /// ```
+///
+/// # Large Files
+///
+/// For security reasons, by default, `Multipart` limits the request body size to 2MB.
+/// See [`DefaultBodyLimit`][default-body-limit] for how to configure this limit.
+///
+/// [default-body-limit]: crate::extract::DefaultBodyLimit
 #[cfg_attr(docsrs, doc(cfg(feature = "multipart")))]
 #[derive(Debug)]
 pub struct Multipart {