Makes Features::from_be_bytes public

Downstream projects building Feature<T> are most likely doing so with a
big-endian byte array, however only `from_le_bytes` is exposed.

lightning/src/ln/features.rs

@@ -718,6 +718,10 @@ impl<T: sealed::Context> Features<T> {
 		Ok(())
 	}
 
+	/// Create a Features given a set of flags, in big-endian. This is in byte order from
+	/// most on-the-wire encodings.
+	///
+	/// This is not exported to bindings users as we don't support export across multiple T
 	fn from_be_bytes(mut flags: Vec<u8>) -> Features<T> {
 		flags.reverse(); // Swap to little-endian
 		Self {