docs: Add missing doc_cfg

ruma-events/src/lib.rs

@@ -113,6 +113,7 @@
 
 #![recursion_limit = "1024"]
 #![warn(missing_debug_implementations, missing_docs, rust_2018_idioms)]
+#![cfg_attr(docsrs, feature(doc_cfg))]
 // This lint is no good
 #![allow(clippy::new_without_default)]
 // Remove this once our MSRV is >= 1.53

ruma-events/src/room/message.rs

@@ -489,6 +489,7 @@ impl TextMessageEventContent {
 
     /// A convenience constructor to create a markdown message.
     #[cfg(feature = "markdown")]
+    #[cfg_attr(docsrs, doc(cfg(feature = "markdown")))]
     pub fn markdown(body: impl Into<String>) -> Self {
         let body = body.into();
         let mut html_body = String::new();