Document opt -S argument.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@81140 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/CommandGuide/opt.pod b/docs/CommandGuide/opt.pod
index 3e23cd1..a6c18b1 100644
--- a/docs/CommandGuide/opt.pod
+++ b/docs/CommandGuide/opt.pod
@@ -51,6 +51,10 @@
 
 Specify the output filename.
 
+=item B<-S>
+
+Write output in LLVM intermediate language (instead of bitcode).
+
 =item B<-{passname}>
 
 B<opt> provides the ability to run any of LLVM's optimization or analysis passes