Update documentation for the -f change.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@79996 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/CommandGuide/opt.pod b/docs/CommandGuide/opt.pod
index 75b7edd..3e23cd1 100644
--- a/docs/CommandGuide/opt.pod
+++ b/docs/CommandGuide/opt.pod
@@ -39,9 +39,9 @@
 
 =item B<-f>
 
-Force overwrite.  Normally, B<opt> will refuse to overwrite an
-output file that already exists.  With this option, B<opt> will
-overwrite the output file and replace it with new bitcode.
+Enable binary output on terminals.  Normally, B<opt> will refuse to
+write raw bitcode output if the output stream is a terminal. With this option,
+B<opt> will write raw bitcode regardless of the output device.
 
 =item B<-help>