doc/dev/documentation: Use appendToVar

Co-authored-by: John Ericson <[email protected]>

Author: roberth

doc/manual/source/development/documentation.md

@@ -244,5 +244,5 @@ $ xdg-open src/external-api-docs/html/index.html
 If you use direnv, or otherwise want to run `configurePhase` in a transient shell, use:
 
 ```bash
-nix-shell -A devShells.x86_64-linux.native-clangStdenv --command 'mesonFlags="$mesonFlags -Ddoc-gen=true"; mesonConfigurePhase'
+nix-shell -A devShells.x86_64-linux.native-clangStdenv --command 'appendToVar mesonFlags "-Ddoc-gen=true"; mesonConfigurePhase'
 ```