doc/go1.16: add multiple release notes for the go command

Added notes for:

* go test -c and -i flags used with unknown flags
* GO111MODULE=on by default
* GOVCS
* Dropped requirements on excluded versions

Removed TODOs for documentation on the retract directive and
'go install pkg@version'. These pages will be written after the beta.

Change-Id: Ic9877a62f908be177a6035a039b72e969e7b7f22
Reviewed-on: https://go-review.googlesource.com/c/go/+/274438
Trust: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

doc/go1.16.html

@@ -80,17 +80,16 @@ Do not send CLs removing the interior tags from such phrases.
 
 <h3 id="go-command">Go command</h3>
 
-<p>
-  TODO
-
-  <!-- CL 237697: https://golang.org/cl/237697: cmd/go: error when -c or -i are used with unknown flags -->
-  <!-- CL 255052: https://golang.org/cl/255052: cmd/go: default to GO111MODULE=on -->
-  <!-- CL 266420: https://golang.org/cl/266420: yes (mention go help vcs): cmd/go: add GOVCS setting to control version control usage -->
-  <!-- CL 244773: https://golang.org/cl/244773: cmd/go/internal/modload: drop requirements on excluded versions -->
-</p>
-
 <h4 id="modules">Modules</h4>
 
+<p><!-- golang.org/issue/41330 -->
+  Module-aware mode is enabled by default, regardless of whether a
+  <code>go.mod</code> file is present in the current working directory or a
+  parent directory. Specifically, the <code>GO111MODULE</code> environment
+  variable now defaults to <code>on</code>. To switch to the previous behavior,
+  set <code>GO111MODULE</code> to <code>auto</code>.
+</p>
+
 <p><!-- golang.org/issue/40728 -->
   Build commands like <code>go</code> <code>build</code> and <code>go</code>
   <code>test</code> no longer modify <code>go.mod</code> and <code>go.sum</code>
@@ -107,9 +106,7 @@ Do not send CLs removing the interior tags from such phrases.
   <code>install</code> to build and install packages in module-aware mode,
   ignoring the <code>go.mod</code> file in the current directory or any parent
   directory, if there is one. This is useful for installing executables without
-  affecting the dependencies of the main module.<br>
+  affecting the dependencies of the main module.
-  TODO: write and link to section in golang.org/ref/mod<br>
-  TODO: write and link to blog post
 </p>
 
 <p><!-- golang.org/issue/40276 -->
@@ -127,8 +124,6 @@ Do not send CLs removing the interior tags from such phrases.
   to indicate that certain published versions of the module should not be used
   by other modules. A module author may retract a version after a severe problem
   is discovered or if the version was published unintentionally.<br>
-  TODO: write and link to section in golang.org/ref/mod<br>
-  TODO: write and link to tutorial or blog post
 </p>
 
 <p><!-- golang.org/issue/26603 -->
@@ -138,6 +133,14 @@ Do not send CLs removing the interior tags from such phrases.
   resolving missing packages.
 </p>
 
+<p><!-- golang.org/issue/36465 -->
+  The <code>go</code> command now ignores requirements on module versions
+  excluded by <code>exclude</code> directives in the main module. Previously,
+  the <code>go</code> command used the next version higher than an excluded
+  version, but that version could change over time, resulting in
+  non-reproducible builds.
+</p>
+
 <h4 id="go-test"><code>go</code> <code>test</code></h4>
 
 <p><!-- golang.org/issue/29062 -->
@@ -150,6 +153,15 @@ Do not send CLs removing the interior tags from such phrases.
   that is still considered to be a passing test.
 </p>
 
+<p><!-- golang.org/issue/39484 -->
+  <code>go</code> <code>test</code> reports an error when the <code>-c</code>
+  or <code>-i</code> flags are used together with unknown flags. Normally,
+  unknown flags are passed to tests, but when <code>-c</code> or <code>-i</code>
+  are used, tests are not run.
+</p>
+
+<h4 id="go-get"><code>go</code> <code>get</code></h4>
+
 <p><!-- golang.org/issue/37519 -->
   The <code>go</code> <code>get</code> <code>-insecure</code> flag is
   deprecated and will be removed in a future version. This flag permits
@@ -161,8 +173,6 @@ Do not send CLs removing the interior tags from such phrases.
   See <code>go</code> <code>help</code> <code>environment</code> for details.
 </p>
 
-<h4 id="go-get"><code>go</code> <code>get</code></h4>
-
 <p><!-- golang.org/cl/263267 -->
   <code>go</code> <code>get</code> <code>example.com/mod@patch</code> now
   requires that some version of <code>example.com/mod</code> already be
@@ -171,6 +181,21 @@ Do not send CLs removing the interior tags from such phrases.
   to patch even newly-added dependencies.)
 </p>
 
+<h4 id="govcs"><code>GOVCS</code> environment variable</h4>
+
+<p><!-- golang.org/issue/266420 -->
+  <code>GOVCS</code> is a new environment variable that limits which version
+  control tools the <code>go</code> command may use to download source code.
+  This mitigates security issues with tools that are typically used in trusted,
+  authenticated environments. By default, <code>git</code> and <code>hg</code>
+  may be used to download code from any repository. <code>svn</code>,
+  <code>bzr</code>, and <code>fossil</code> may only be used to download code
+  from repositories with module paths or package paths matching patterns in
+  the <code>GOPRIVATE</code> environment variable. See
+  <a href="/cmd/go/#hdr-Controlling_version_control_with_GOVCS"><code>go</code>
+  <code>help</code> <code>vcs</code></a> for details.
+</p>
+
 <h4 id="all-pattern">The <code>all</code> pattern</h4>
 
 <p><!-- golang.org/cl/240623 -->