rephrase and show default setting value

Author: vindarel

content/en/usage/common_lisp.md

@@ -124,7 +124,8 @@ all known symbols of the current Lisp image*, and it brings you to
 this symbol's definition.
 
 > If you can't find the definition of built-in symbols provided by your implementation, and get the `directory does not exists: ...` error, then you may need to specify the `source directory` of your implementation.
-> For example, if you are using `roswell` to start your `lisp repl`, you can create the `~/.roswell/init.lisp`, and add `(sb-ext:set-sbcl-source-location "/path/to/your/source-of-implementation/")` to specify the source direcoty. (Taken `sbcl` implementation for example.)
+> Add this in your init file: `(sb-ext:set-sbcl-source-location "/path/to/your/source-of-implementation/")` to specify the source direcoty (given `sbcl` is the underlying implementation). A usual value is `"/usr/share/sbcl-source/"`.
+> If you installed Lem with Roswell (which isn't the preferred method anymore), you can create the `~/.roswell/init.lisp` init file, and you can install SBCL source files with `ros install sbcl --without-install`.
 
 "All symbols" really means all the symbols of any package that was
 loaded in the image. This includes the implementation and Lem's

content/zh-TW/usage/common_lisp.md

@@ -100,7 +100,8 @@ all known symbols of the current Lisp image*, and it brings you to
 this symbol's definition.
 
 > If you can't find the definition of built-in symbols provided by your implementation, and get the `directory does not exists: ...` error, then you may need to specify the `source directory` of your implementation.
-> For example, if you are using `roswell` to start your `lisp repl`, you can create the `~/.roswell/init.lisp`, and add `(sb-ext:set-sbcl-source-location "/path/to/your/source-of-implementation/")` to specify the source direcoty. (Taken `sbcl` implementation for example.)
+> Add this in your init file: `(sb-ext:set-sbcl-source-location "/path/to/your/source-of-implementation/")` to specify the source direcoty (given `sbcl` is the underlying implementation). A usual value is `"/usr/share/sbcl-source/"`.
+> If you installed Lem with Roswell (which isn't the preferred method anymore), you can create the `~/.roswell/init.lisp` init file, and you can install SBCL source files with `ros install sbcl --without-install`.
 
 "All symbols" really means all the symbols of any package that was
 loaded in the image. This includes the implementation and Lem's