Fix/clarify dirname/basename docs wrt. locales

For basename(), we declare the behavior regarding invalid characters in
the path as being undefined, since that depends on the availability of
mblen, and also on the position of the invalid characters prior to PHP
8.0.0[1].

dirname() is actually not local-aware, but relies on an ASCII
compatible character encoding regarding the directory separator.  On
Windows, it is however, dependent on the currently set codepage
(although a fallback is still in place to use the Windows ANSI codepage
of the operating system[2], if the string is not valid for the current
codepage).

Again, we declared failure to comply to these assumptions as resulting
in undefined behavior.  Users should make sure to pass valid strings.

[1] <http://git.php.net/?p=php-src.git;a=commitdiff;h=90705d44e3da1d0aa7b8b4fd921ec597391eccb2>
[2] <https://github.com/php/php-src/blob/5e015425263c28d40fd49ee386135f02d0e76975/win32/codepage.h#L95-L106>

reference/filesystem/functions/basename.xml

@@ -29,6 +29,8 @@
     <function>basename</function> is locale aware, so for it to see the 
     correct basename with multibyte character paths, the matching locale must 
     be set using the <function>setlocale</function> function.
+    If <parameter>path</parameter> contains characters which are invalid for the
+    current locale, the behavior of <function>basename</function> is undefined.
    </para>
   </caution>
  </refsect1>

reference/filesystem/functions/dirname.xml

@@ -27,9 +27,16 @@
   </note>
   <caution>
    <para>
-    <function>dirname</function> is locale aware, so for it to see the 
-    correct directory name with multibyte character paths, the matching locale must 
-    be set using the <function>setlocale</function> function.
+    On Windows, <function>dirname</function> assumes the currently set codepage, so for it to see the 
+    correct directory name with multibyte character paths, the matching codepage must 
+    be set.
+    If <parameter>path</parameter> contains characters which are invalid for the
+    current codepage, the behavior of <function>basename</function> is undefined.
+   </para>
+   <para>
+    On other systems, <function>dirname</function> assumes <parameter>path</parameter>
+    to be encoded in an ASCII compatible encoding. Otherwise the behavior of the
+    the function is undefined.
    </para>
   </caution>
  </refsect1>