[skip ci] Add info on how to check for PHP NULL when using the ZPP callable check

Author: Girgias

docs/parameter-parsing-api.md

@@ -100,6 +100,9 @@ The following characters also have a meaning in the specifier string:
   zend_bool* must be passed after the corresponding bool*, zend_long* or
   double* arguments, respectively. A non-zero value will be written to the
   zend_bool if a PHP NULL is passed.
+  For `f` use the ``ZEND_FCI_INITIALIZED(fci)`` macro to check if a callable
+  has been provided and ``!ZEND_FCI_INITIALIZED(fci)`` to check if a PHP NULL
+  is passed.
 
 ## Note on 64bit compatibility