Logging functions: level parameter is not optional

This parameter cannot be optional, as the following parameter is mandatory.

Also completed the corresponding documentation.
diff --git a/tests/mocks/core/common.php b/tests/mocks/core/common.php
index 24d645a..0ccfe1e 100644
--- a/tests/mocks/core/common.php
+++ b/tests/mocks/core/common.php
@@ -178,7 +178,7 @@
 
 if ( ! function_exists('log_message'))
 {
-	function log_message($level = 'error', $message, $php_error = FALSE)
+	function log_message($level, $message, $php_error = FALSE)
 	{
 		return TRUE;
 	}