docs: fix docsrs builds with the fs feature enabled (#6585)

tokio/src/doc/mod.rs

@@ -43,5 +43,5 @@ impl mio::event::Source for NotDefinedHere {
     }
 }
 
-#[cfg(feature = "net")]
+#[cfg(any(feature = "net", feature = "fs"))]
 pub mod os;

tokio/src/lib.rs

@@ -628,7 +628,7 @@ pub mod stream {}
 #[cfg(docsrs)]
 pub mod doc;
 
-#[cfg(feature = "net")]
+#[cfg(any(feature = "net", feature = "fs"))]
 #[cfg(docsrs)]
 #[allow(unused)]
 pub(crate) use self::doc::os;