doc: Add comment for fetchurl for name & url

fetchurl can be given a name and url aside from just the url.
Giving a name can be useful if the url has invalid characters such as
tilde for the store.

src/libexpr/primops/fetchTree.cc

@@ -529,9 +529,20 @@ static void prim_fetchurl(EvalState & state, const PosIdx pos, Value * * args, V
 
 static RegisterPrimOp primop_fetchurl({
     .name = "__fetchurl",
-    .args = {"url"},
+    .args = {"args"},
     .doc = R"(
-      Download the specified URL and return the path of the downloaded file.
+      If args is a URL, return the path of the downloaded file.
+      Otherwise, it can be an attribute with the following attributes
+      (all except url are optional):
+
+      - `url`
+
+        The URL of the file to download.
+
+      - `name` (default: `url without the protocol`)
+
+        A name for the file in the store. This can be useful if the URL has any
+        characters that are invalid for the store.
 
       Not available in [restricted evaluation mode](@docroot@/command-ref/conf-file.md#conf-restrict-eval).
     )",