Document --sandbox_debug in the user manual.
@@ -1415,6 +1415,13 @@ either to the terminal, or to additional log files.
which drive a build.
</p>
+<h4 id='flag--sandbox_debug'><code class='flag'>--sandbox_debug</code></h4>
+<p>
+ This option causes Bazel to print extra debugging information when using sandboxing for action
+ execution. This option also preserves sandbox directories, so that the files visible to actions
+ during execution can be examined.
+</p>
+
<h4 id='flag--subcommands'><code class='flag'>--subcommands</code> (<code>-s</code>)</h4>
<p>
This option causes Bazel's execution phase to print the full command line
