doc: Contributing tools/checkpatch.sh example update.

* Add `./tools/checkpatch.sh -c -u -m -g HEAD~...HEAD` example
  to match checks performed by our CI.
* Add cross-reference to checkpatch.sh documentation.

Signed-off-by: Tomasz 'CeDeROM' CEDRO <tomek@cedro.info>

Author: cederom

Documentation/components/tools/checkpatch.rst

@@ -1,3 +1,5 @@
+.. _checkpatch.sh:
+
 =================
 ``checkpatch.sh``
 =================

Documentation/contributing/coding_style.rst

@@ -8,28 +8,29 @@ C Coding Standard
 NuttX follows a specific coding style which needs to be followed at all times
 a contribution to be accepted. Please read this document before working on
 new code so that you can follow the style from the start. To check your code
-for conformance to the coding style, you should use the checkpatch.sh script
-(that calls the `nxstyle <#nxstyle>`_ tool) included under ``tools/`` in the
-main NuttX repository, or enable the pre-commit functionality described in
-`pre-commit <#precommit>`__.
+for conformance to the coding style, you should use the :ref:`checkpatch.sh`
+script (that calls the `nxstyle <#nxstyle>`_ tool) included under ``tools/``
+in the main NuttX repository, or enable the pre-commit functionality
+described in `pre-commit <#precommit>`__.
 
 **************************
 Quick Check for Compliance
 **************************
 
-You should check for coding style issues before submitting your Pull Request.
-There is a script that you can run to check for coding styles issue:
+Please verify formatting of your git commits before a Pull Request is
+submitted/updated. Use :ref:`checkpatch.sh` script that will verify the coding
+style, encoding, and spelling issues:
 
   .. code-block:: bash
 
-    ./tools/checkpatch.sh -g HEAD~...HEAD
+    ./tools/checkpatch.sh -c -u -m -g HEAD~...HEAD
 
 Alternatevily you can run this script passing the .c file or .h header you
-want to check:
+want to check using ``-f`` switch:
 
   .. code-block:: bash
 
-    ./tools/checkpatch.sh -f path/to/your/file.c
+    ./tools/checkpatch.sh -c -u -m -f path/to/your/file.c
 
 *******************
 General Conventions