Correct NIX_BUILD_PATH default description

Source: 067076287b/src/nix-build/nix-build.cc (L362-L381)

doc/manual/src/command-ref/nix-shell.md

@@ -101,7 +101,8 @@ The following common options are supported:
 
   - `NIX_BUILD_SHELL`\
     Shell used to start the interactive environment. Defaults to the
-    `bash` found in `PATH`.
+    `bash` found in `<nixpkgs>`, falling back to the `bash` found in
+    `PATH` if not found.
 
 # Examples