Update documentation for the -f change.

llvm-svn: 79996
diff --git a/llvm/docs/CommandGuide/llvm-link.pod b/llvm/docs/CommandGuide/llvm-link.pod
index 5f4dcb6..8a2a8c5 100644
--- a/llvm/docs/CommandGuide/llvm-link.pod
+++ b/llvm/docs/CommandGuide/llvm-link.pod
@@ -33,8 +33,9 @@
 
 =item B<-f>
 
-Overwrite output files.  By default, B<llvm-link> will not overwrite an output
-file if it already exists.
+Enable binary output on terminals.  Normally, B<llvm-link> will refuse to
+write raw bitcode output if the output stream is a terminal. With this option,
+B<llvm-link> will write raw bitcode regardless of the output device.
 
 =item B<-o> F<filename>