Document --action_env in the user manual.

Fixes https://github.com/bazelbuild/bazel/issues/6264

RELNOTES:
PiperOrigin-RevId: 310926775
diff --git a/site/docs/user-manual.html b/site/docs/user-manual.html
index edb36aa..e1b9b92 100644
--- a/site/docs/user-manual.html
+++ b/site/docs/user-manual.html
@@ -491,6 +491,18 @@
   used CROSSTOOL file.
 </p>
 
+<h4 id='flag--action_env'>
+  <code class='flag'>--action_env=<var>VAR=VALUE</var></code>
+</h4>
+<p>
+  Specifies the set of environment variables available during the execution of all actions.
+  Variables can be either specified by name, in which case the value will be taken from the
+  invocation environment, or by the `name=value` pair which sets the value independent of the
+  invocation environment.
+
+  This `--action_env` flag can be specified multiple times. If a value is assigned to the same
+  variable across multiple `--action_env` flags, the latest assignment wins.
+</p>
 
 <h4 id='flag--experimental_action_listener'>
   <code class='flag'>--experimental_action_listener=<var>label</var></code>