Updated documentation re: no equals sign and single quote

NEWS

@@ -6,7 +6,8 @@ This file documents the major additions and syntax changes between releases.
 	Fix check_ircd binding to wrong interface (#668778)
 	Add proxy-authorization option to check_http (Marcel Kuiper - #1323230, Bryan Irvine - #2863925)
 	WARNINGS
-	Updated developer documentation to say that performance labels should not have an equals sign in the label
+	Updated developer documentation to say that performance labels should not have an equals sign or
+	single quote in the label
 
 1.4.14 16th September 2009
 	check_http has options to specify the HTTP method (#2155152)

doc/developer-guidelines.sgml

@@ -333,10 +333,10 @@
 		<orderedlist>
 		<listitem><para>space separated list of label/value pairs</para>
 			</listitem>
-		<listitem><para>label can contain any characters</para>
+		<listitem><para>label can contain any characters except the equals sign or single quote (')</para>
 			</listitem>
 		<listitem><para>the single quotes for the label are optional. Required if 
-			spaces, = or ' are in the label</para>
+			spaces are in the label</para>
 			</listitem>
 		<listitem><para>label length is arbitrary, but ideally the first 19 characters
 			are unique (due to a limitation in RRD). Be aware of a limitation in the