doc/go1.11: explain new vet typechecking behaviour in release notes

Since Go1.10, go test runs vet on the tests before executing them.

Moreover, the vet tool typechecks the package under analysis with
go/types before running. In Go1.10, a typechecking failure just caused
a warning to be printed. In Go1.11, a typechecking failure will cause
vet to exit with a fatal error (see Issue #21287).

This means that starting with Go1.11, tests that don't typecheck will
fail immediately. This would not normally be an issue, since a test
that doesn't typecheck shouldn't even compile, and it should already
be broken.

Unfortunately, there's a bug in gc that makes it accept programs with
unused variables inside a closure (Issue #3059). This means that a
test with an unused variable inside a closure, that compiled and
passed in Go1.10, will fail in the typechecking step of vet starting
with Go1.11.

Explain this in the 1.11 release notes.

Fixes #26109

Change-Id: I970c1033ab6bc985d8c64bd24f56e854af155f96
Reviewed-on: https://go-review.googlesource.com/121455Reviewed-by: Ian Lance Taylor <iant@golang.org>

doc/go1.11.html

@@ -157,6 +157,35 @@ Do not send CLs removing the interior tags from such phrases.
   please <a href="https://golang.org/issue/new">file an issue</a> to let us know about them.
 </p>
 
+<h2 id="tools">Tools</h2>
+
+<h3 id="test">Test</h3>
+
+<p>
+  Since Go1.10, the <code>go</code>&nbsp;<code>test</code> command runs
+  <code>go</code>&nbsp;<code>vet</code> on the package being tested,
+  to identify problems before running the test. Since <code>vet</code>
+  typechecks the code with <a href="/pkg/go/types/">go/types</a>
+  before running, tests that do not typecheck will now fail.
+
+  In particular, tests that contain an unused variable inside a
+  closure compiled with Go1.10, because the Go compiler incorrectly
+  accepted them (<a href="https://golang.org/issues/3059">Issue #3059</a>),
+  but will now fail, since <code>go/types</code> correctly reports an
+  "unused variable" error in this case.
+</p>
+
+
+<h3 id="vet">Vet</h3>
+
+<p><!-- CL 108555 -->
+  The <a href="/cmd/vet/"><code>go</code>&nbsp;<code>vet</code></a>
+  command now reports a fatal error when the package under analysis
+  does not typecheck. Previously, a type checking error simply caused
+  a warning to be printed, and <code>vet</code> to exit with status 1.
+</p>
+
+
 <h2 id="library">Core library</h2>
 
 <p>