Add doc page for new control syntax

Author: odersky

docs/docs/reference/other-new-features/control-syntax.md

@@ -0,0 +1,53 @@
+---
+layout: doc-page
+title: New Control Syntax
+---
+
+Scala 3 has a new "quiet" syntax for control expressions that does not rely in
+enclosing the condition in parentheses, and also allows to drop parentheses or braces
+around the generators of a `for`-expression. Examples:
+```scala
+if x < 0 then -x else x
+
+while x >= 0 do x = f(x)
+
+for x <- xs if x > 0
+yield x * x
+
+for
+  x <- xs
+  y <- ys
+do
+  println(x + y)
+```
+
+The rules in detail are:
+
+ - The condition of an `if`-expression can be written without enclosing parentheses if it is followed by a `then`.
+ - The condition of a `while`-loop can be written without enclosing parentheses if it is followed by a `do`.
+ - The enumerators of a `for`-expression can be written without enclosing parentheses or braces if they are followed by a `yield` or `do`.
+ - A `do` in a `for`-expression expresses a `for`-loop.
+ - Newline characters are not statement separators in a condition of an `if` or a `while`.
+   So the meaning of newlines is the same no matter whether parentheses are present
+   or absent.
+ - Newline characters are statement separators in the enumerators of a `for`-expression if and only if the first generator of the `for` expression appears on a new line.
+
+To illustrate the last rule above, compare
+```scala
+  for x <- xs
+    ++ ys      // newline not significant here
+    ++ zs      // newline not significant here
+  do println(x)
+```
+and
+```scala
+  for
+    x <- xs
+    y <- xs     // newline is significant here
+  do println(x)
+```
+
+### Rewrites
+
+The Dotty compiler can rewrite source code from old syntax and new syntax and back.
+When invoked with options `-rewrite -new-syntax` it will rewrite from old to new syntax, dropping parentheses and braces in conditions and enumerators. When invoked with with options `-rewrite -old-syntax` it will rewrite in the reverse direction, inserting parentheses and braces as needed.

docs/sidebar.yml

@@ -101,6 +101,8 @@ sidebar:
               url: docs/reference/other-new-features/tupled-function.html
             - title: threadUnsafe Annotation
               url: docs/reference/other-new-features/threadUnsafe-annotation.html
+            - title: New Control Syntax
+              url: docs/reference/other-new-features/control-syntax.html
         - title: Other Changed Features
           subsection:
             - title: Structural Types