Describe how non-ASCII variable names behave instead of asserting they are unsupported

language/variables.xml

@@ -23,10 +23,22 @@
 
    <note>
     <simpara>
-     PHP doesn't support Unicode variable names, however, some character
-     encodings (such as UTF-8) encode characters in such a way that all bytes
-     of a multi-byte character fall within the allowed range, thus making it a
-     valid variable name.
+     Variable names are compared as bytes, not as characters. PHP does not
+     interpret or validate the encoding of the bytes from 128 through 255, and
+     two names refer to the same variable only when their bytes are identical.
+    </simpara>
+    <simpara>
+     One consequence is that names written in UTF-8 work, because every byte of
+     a UTF-8 multi-byte sequence falls inside the accepted range. The same holds
+     for any encoding whose non-ASCII bytes all fall in that range, and for byte
+     sequences that are not valid text in any encoding. Encodings that use bytes
+     below 128 within a multi-byte sequence, such as Shift-JIS or UTF-16, do not.
+    </simpara>
+    <simpara>
+     Another is that two names which display identically may be distinct.
+     <literal>ä</literal> encoded as U+00E4 and as <literal>a</literal>
+     followed by the combining diaeresis U+0308 are different variables, as are
+     a name with and without a trailing U+00A0 NO-BREAK SPACE.
     </simpara>
    </note>