Add minimumVisibility property to FunctionCommentSniff

Author: javer

README.md

@@ -89,6 +89,7 @@ This sniff provides the option:
 <rule ref="Swivl.Commenting.FunctionComment">
     <properties>
         <property name="requiredPhpdoc" value="always"/>
+        <property name="minimumVisibility" value="private"/>
     </properties>
 </rule>
 ```

Swivl/Sniffs/Commenting/FunctionCommentSniff.php

@@ -21,6 +21,8 @@ class FunctionCommentSniff extends SquizFunctionCommentSniff
 
     public $requiredPhpdoc = self::REQUIRED_PHPDOC_BY_MAP;
 
+    public $minimumVisibility = 'private';
+
     /**
      * @var DocCommentSniff
      */