Add reference doc page

Author: odersky

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

@@ -1945,7 +1945,7 @@ object Parsers {
         mkIf(cond, thenp, elsep)
       }
 
-    /**    MatchClause ::= `match' [‘inline’] `{' CaseClauses `}'
+    /**    MatchClause ::= `match' `{' CaseClauses `}'
      */
     def matchClause(t: Tree): Match =
       in.endMarkerScope(MATCH) {

docs/docs/reference/changed-features/match-syntax.md

@@ -0,0 +1,43 @@
+---
+layout: doc-page
+title: Match Expressions
+---
+
+The syntactical precedence of match expressions has been changed.
+`match` is still a keyword, but it is used like an alphabetical operator. This has several consequences:
+
+ 1. `match` expressions can be chained:
+
+    ```scala
+    xs match {
+      case Nil => "empty"
+      case x :: xs1 => "nonempty"
+    } match {
+      case "empty" => 0
+      case "nonempty" => 1
+    }
+
+ 2. `match` may follow a period:
+
+     ```scala
+     if xsDefined
+        && xs.match {
+             case Nil => false
+             case _ => true
+           }
+     then "nonempty"
+     else "empty"
+
+ 3. The scrutinee of a match expression must be an `InfixExpr`. Previously the scrutinee could be followed by a type ascription `: T`, but this is no longer supported. So `x : T match { ... }` now has to be
+ written `(x: T) match { ... }`.
+
+## Syntax
+
+The new syntax of match expressions is as follows.
+```
+InfixExpr    ::=  ...
+               |  InfixExpr MatchClause
+SimpleExpr   ::=  ...
+               |  SimpleExpr ‘.’ MatchClause
+MatchClause  ::=  ‘match’ ‘{’ CaseClauses ‘}’
+```

docs/sidebar.yml

@@ -127,6 +127,8 @@ sidebar:
               url: docs/reference/changed-features/implicit-conversions.html
             - title: Overload Resolution
               url: docs/reference/changed-features/overload-resolution.html
+            - title: Match Expressions
+              url: docs/reference/changed-features/match-syntax.html
             - title: Vararg Patterns
               url: docs/reference/changed-features/vararg-patterns.html
             - title: Pattern Bindings