improve to_qr_string doc

src/util/address.rs

@@ -353,7 +353,11 @@ impl Address {
         self.payload.script_pubkey()
     }
 
-    /// Creates a string optimized to be encoded in QR codes, meaning it becomes uppercase if bech32
+    /// Creates a string optimized to be encoded in QR codes, meaning it becomes uppercase if bech32.
+    /// Quoting BIP 173 "inside QR codes uppercase SHOULD be used, as those permit the use of
+    /// alphanumeric mode, which is 45% more compact than the normal byte mode."
+    /// Even inside Bitcoin URI may be more efficient to use the uppercase address since in QR codes
+    /// encoding modes can be mixed as needed within a QR symbol.
     pub fn to_qr_string(&self) -> String {
         let address_string = self.to_string();
         match self.payload {