docs: Mark relay_fee as deprecated in protocol v1.6+

Add documentation note that `relay_fee` is deprecated in protocol v1.6
and users should use `mempool_get_info` instead.

Co-Authored-By: Claude Code AI

Author: tnull

src/api.rs

@@ -308,6 +308,9 @@ pub trait ElectrumApi {
     fn estimate_fee(&self, number: usize, mode: Option<EstimationMode>) -> Result<f64, Error>;
 
     /// Returns the minimum accepted fee by the server's node in **Bitcoin, not Satoshi**.
+    ///
+    /// **Note:** This method is deprecated in protocol v1.6+. Use [`mempool_get_info`](#method.mempool_get_info)
+    /// instead, which provides `minrelaytxfee` along with additional mempool fee information.
     fn relay_fee(&self) -> Result<f64, Error>;
 
     /// Subscribes to notifications for activity on a specific *scriptPubKey*.