_content/doc: convert go1 release notes to markdown

The first set of release notes is a template.
I converted it as follows:

  1. Run html2md.
  2. That escaped ` and * in the template function calls, so
     I manually removed the escapes.
  3. I made some small tweaks to avoid `<` being double-escaped
     so it rendered as `&lt;`.

The remaining diffs are due to "fancy" quotation marks and
apostrophes, as mentioned in the previous CL.

Change-Id: I085f5d141d63fbb751bffeec4dca760d1fd15464
Reviewed-on: https://go-review.googlesource.com/c/website/+/540295
Run-TryBot: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Heschi Kreinick <heschi@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

_content/doc/progs/go1.go

@@ -140,7 +140,7 @@ func compositeLiterals() {
 }
 
 func runeType() {
-	// STARTRUNE OMIT
+	l            // STARTRUNE OMIT
 	delta := 'δ' // delta has type rune.
 	var DELTA rune
 	DELTA = unicode.ToUpper(delta)
@@ -243,3 +243,26 @@ func osIsExist() {
 	}
 	_ = f
 }
+
+func closeExample() {
+	// STARTCLOSE OMIT
+	var c chan int
+	var csend chan<- int = c
+	var crecv <-chan int = c
+	close(c)     // legal
+	close(csend) // legal
+	close(crecv) // illegal
+	// ENDCLOSE OMIT
+}
+
+func Bug() (i, j, k int) {
+	for i = 0; i < 5; i++ {
+		for j := 0; j < 5; j++ { // Redeclares j.
+			k += i * j
+			if k > 100 {
+				return // Rejected: j is shadowed here.
+			}
+		}
+	}
+	return // OK: j is not shadowed here.
+}