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

Consider the example:
```scala
for {
  x <- 1 to 5
  if true; if true; if true
} yield x
```
Note, that multiple if guards start from a new line and are not separated with a semicolon or a new line.
This code is compiled ok in Scala 3 and is a valid code according to Scala 2 syntax reference:
(though looks like there is a bug in implementation scala/bug#12323)

But it's not supported according to the Scala 3 syntax reference.

related issues:
scala/bug#12323
https://youtrack.jetbrains.com/issue/SCL-13220

Author: unkarjedy

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -2459,7 +2459,7 @@ object Parsers {
       else Nil
 
     /** Enumerator  ::=  Generator
-     *                |  Guard
+     *                |  Guard {Guard}
      *                |  Pattern1 `=' Expr
      */
     def enumerator(): Tree =

docs/docs/internals/syntax.md

@@ -269,9 +269,9 @@ 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)
+Generator         ::=  [‘case’] Pattern1 ‘<-’ Expr                              GenFrom(pat, expr)
 Guard             ::=  ‘if’ PostfixExpr
 
 CaseClauses       ::=  CaseClause { CaseClause }                                Match(EmptyTree, cases)

docs/docs/reference/syntax.md

@@ -264,9 +264,9 @@ ForExpr           ::=  ‘for’ (‘(’ Enumerators ‘)’ | ‘{’ Enumerat
                     |  ‘for’ Enumerators (‘do’ Expr | ‘yield’ Expr)
 Enumerators       ::=  Generator {semi Enumerator | Guard}
 Enumerator        ::=  Generator
-                    |  Guard
-                    |  Pattern1 ‘=’ Expr
-Generator         ::=  [‘case’] Pattern1 ‘<-’ Expr
+                    |  Guard {Guard}
+                    |  Pattern1 ‘=’ Expr                                        GenAlias(pat, expr)
+Generator         ::=  [‘case’] Pattern1 ‘<-’ Expr                                       GenFrom(pat, expr)
 Guard             ::=  ‘if’ PostfixExpr
 
 CaseClauses       ::=  CaseClause { CaseClause }