reference/syntax.md: support multiple if-guards on same line

Author: unkarjedy

docs/docs/reference/syntax.md

@@ -258,7 +258,7 @@ ForExpr           ::=  ‘for’ (‘(’ Enumerators ‘)’ | ‘{’ Enumerat
                     |  ‘for’ Enumerators (‘do’ Expr | ‘yield’ Expr)             ForDo(enums, expr)
 Enumerators       ::=  Generator {semi Enumerator | Guard}
 Enumerator        ::=  Generator
-                    |  Guard
+                    |  Guard {Guard}
                     |  Pattern1 ‘=’ Expr                                        GenAlias(pat, expr)
 Generator         ::=  [‘case’] Pattern1 ‘<-’ Expr                                       GenFrom(pat, expr)
 Guard             ::=  ‘if’ PostfixExpr