Update doc/build-unix.md

Co-authored-by: UdjinM6 <UdjinM6@users.noreply.github.com>

Author: PastaPastaPasta

doc/build-unix.md

@@ -61,6 +61,8 @@ Now, you can either build from self-compiled [depends](/depends/README.md) or in
 sudo apt-get install libbacktrace-dev libevent-dev libboost-dev
 ```
 
+Note: libbacktrace-dev is available in Debian 13 (Trixie) and Ubuntu 25.04+.
+For older releases, use the /depends/README.md which includes all required libraries.
 SQLite is required for the descriptor wallet:
 
 ```sh