doc: improve ninja build for --built-in-modules-path

The current ninja build does not work with `--node-builtin-modules-path`
flag without passing `--ninja` as it will use `make` to build from
scratch again.

doc/contributing/building-node-with-ninja.md

@@ -32,6 +32,12 @@ make -j4 # With this flag, Ninja will limit itself to 4 parallel jobs,
          # regardless of the number of cores on the current machine.
 ```
 
+If you wish to only modify JS layer in `lib`, you can use:
+
+```bash
+./configure --ninja --node-builtin-modules-path "$(pwd)"
+```
+
 ## Producing a debug build
 
 To create a debug build rather than a release build: