Allow negative coordinates in MonitorRules

Monitor/output position (-1, -1) remains as a single indicator value for autoconfigure layout.
Additionally, one minor comment typo is corrected.

config.def.h

@@ -36,6 +36,8 @@ static const Layout layouts[] = {
 };
 
 /* monitors */
+/* (x=-1, y=-1) is reserved as an "autoconfigure" monitor position indicator */
+/* WARNING: negative values other than (-1, -1) cause problems with xwayland clients' menus */
 /* NOTE: ALWAYS add a fallback rule, even if you are completely sure it won't be used */
 static const MonitorRule monrules[] = {
 	/* name       mfact  nmaster scale layout       rotate/reflect                x    y */