docs: discuss unique paths for #[server] functions (#1610)

docs/book/src/server/25_server_functions.md

@@ -70,6 +70,18 @@ There are a few things to note about the way you define a server function, too.
 - We provide the macro a path. This is a prefix for the path at which we’ll mount a server function handler on our server. (See examples for [Actix](https://github.com/leptos-rs/leptos/blob/main/examples/todo_app_sqlite/src/main.rs#L44) and [Axum](https://github.com/leptos-rs/leptos/blob/598523cd9d0d775b017cb721e41ebae9349f01e2/examples/todo_app_sqlite_axum/src/main.rs#L51).)
 - You’ll need to have `serde` as a dependency with the `derive` featured enabled for the macro to work properly. You can easily add it to `Cargo.toml` with `cargo add serde --features=derive`.
 
+## Server Function URL Prefixes
+
+You can optionally define a specific URL prefix to be used in the definition of the server function.
+This is done by providing an optional 2nd argument to the `#[server]` macro.
+By default the URL prefix will be `/api`, if not specified.
+Here are some examples:
+
+```rust
+#[server(AddTodo)]         // will use the default URL prefix of `/api`
+#[server(AddTodo, "/foo")] // will use the URL prefix of `/foo`
+```
+
 ## Server Function Encodings
 
 By default, the server function call is a `POST` request that serializes the arguments as URL-encoded form data in the body of the request. (This means that server functions can be called from HTML forms, which we’ll see in a future chapter.) But there are a few other methods supported. Optionally, we can provide another argument to the `#[server]` macro to specify an alternate encoding:
@@ -105,6 +117,21 @@ In other words, you have two choices:
 >
 > The CBOR encoding is suported for historical reasons; an earlier version of server functions used a URL encoding that didn’t support nested objects like structs or vectors as server function arguments, which CBOR did. But note that the CBOR forms encounter the same issue as `PUT`, `DELETE`, or JSON: they do not degrade gracefully if the WASM version of your app is not available.
 
+
+## Server Functions Endpoint Paths
+
+By default, a unique path will be generated. You can optionally define a specific endpoint path to be used in the URL. This is done by providing an optional 4th argument to the `#[server]` macro. Leptos will generate the complete path by concatenating the URL prefix (2nd argument) and the endpoint path (4th argument).
+For example,
+
+```rust
+#[server(MyServerFnType, "/api", "Url", "hello")]
+```
+will generate a server function endpoint at `/api/hello` that accepts a POST request.
+
+> **Can I use the same server function endpoint path with multiple encodings?**
+>
+> No. Different server functions must have unique paths. The `#[server]` macro automatically generates unique paths, but you need to be careful if you choose to specify the complete path manually, as the server looks up server functions by their path.
+
 ## An Important Note on Security
 
 Server functions are a cool technology, but it’s very important to remember. **Server functions are not magic; they’re syntax sugar for defining a public API.** The _body_ of a server function is never made public; it’s just part of your server binary. But the server function is a publicly accessible API endpoint, and it’s return value is just a JSON or similar blob. You should _never_ return something sensitive from a server function.

leptos_macro/src/lib.rs

@@ -818,6 +818,10 @@ pub fn slot(args: proc_macro::TokenStream, s: TokenStream) -> TokenStream {
 /// - Your server must be ready to handle the server functions at the API prefix you list. The easiest way to do this
 ///   is to use the `handle_server_fns` function from [`leptos_actix`](https://docs.rs/leptos_actix/latest/leptos_actix/fn.handle_server_fns.html)
 ///   or [`leptos_axum`](https://docs.rs/leptos_axum/latest/leptos_axum/fn.handle_server_fns.html).
+/// - **Server functions must have unique paths**. Unique paths are automatically generated for each 
+///   server function. If you choose to specify a path in the fourth argument, you must ensure that these
+///   are unique. You cannot define two server functions with the same URL prefix and endpoint path,
+///   even if they have different URL encodings, e.g. a POST method at `/api/foo` and a GET method at `/api/foo`.
 ///
 /// ## Server Function Encodings
 ///