Mention chroot in --batch description.

Fixes #134.

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MOS_MIGRATED_REVID=103935102
diff --git a/site/docs/bazel-user-manual.html b/site/docs/bazel-user-manual.html
index 4aa6535..1636db1 100644
--- a/site/docs/bazel-user-manual.html
+++ b/site/docs/bazel-user-manual.html
@@ -3451,7 +3451,8 @@
   This switch will cause bazel to be run in batch mode, instead of the
   standard client/server mode described <a href='#client/server'>above</a>.
   Doing so provides more predictable semantics with respect to signal handling,
-  job control, and environment variable inheritance.
+  job control, and environment variable inheritance, and is necessary for running
+  bazel in a chroot jail.
 </p>
 
 <p>