Merge pull request #2162 from MGatner/common-docs

User Guide updates for Common.php

Author: lonnieezell

user_guide_src/source/extending/common.rst

@@ -0,0 +1,13 @@
+**************************
+Replacing Common Functions
+**************************
+
+There are quite a few functions necessary to CodeIgniter that need to be loaded early for use in the core classes and
+thus cannot be placed into a helper. While most users will never have any need to do this, but the option to replace
+these functions does exist for those who would like to significantly alter the CodeIgniter core. In the ``App\``
+directory there is a file ``Common.php``, and any functions defined in there will take precedence over the versions
+found in ``system/Common.php``. This is also an opportunity to create globally-available functions you intend to
+use throughout the framework.
+
+.. note:: Messing with a core system class has a lot of implications, so make sure you know what you are doing before
+    attempting it.

user_guide_src/source/extending/index.rst

@@ -8,6 +8,7 @@ CodeIgniter 4 has been designed to be easy to extend or build upon.
     :titlesonly:
 
     core_classes
+    common
     events
     basecontroller
     contributing