kate: add note about &rustIdent;

src/etc/kate/rust.xml

@@ -1,6 +1,9 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!DOCTYPE language SYSTEM "language.dtd"
 [
+	<!-- TODO: Kate's regex engine has very limited support for
+	predefined char classes, so making rustIdent consistent with actual
+	Rust identifiers will be a bit difficult -->
 	<!ENTITY rustIdent "[a-zA-Z_][a-zA-Z_0-9]*">
 ]>
 <language name="Rust" version="0.6" kateversion="2.4" section="Sources" extensions="*.rs;*.rc" mimetype="text/x-rust" priority="15">