Mention symlinks for is_file() functions (#3924)

Fixes GH-3719

Author: vera-ju

reference/filesystem/functions/is-file.xml

@@ -13,7 +13,8 @@
    <methodparam><type>string</type><parameter>filename</parameter></methodparam>
   </methodsynopsis>
   <para>
-   Tells whether the given file is a regular file.
+   Tells whether the given file is a regular file. If <parameter>filename</parameter> is a
+   symbolic link, it will resolve the symlink and will give information about the referred file.
   </para>
  </refsect1>