Fix LTO document

[Prazek]

Summary

fixed very old document

[mehdi_amini]

LGTM.

docs/LinkTimeOptimization.rst

92

Note: this is a no-op on OS X for instance, but it does not hurt either.

[Prazek]

docs/LinkTimeOptimization.rst

92

but on linux it doesn't work without it (at least on my setup)

[mehdi_amini]

r274811

(Somehow you referenced the wrong revision in the commit, closing this manually)

[Prazek]

could be, arc was doing some weird things

docs/LinkTimeOptimization.rst

.. code-block:: c++
   int main() {
     return foo1();
   }
 
 To compile, run:
 
 .. code-block:: console
 
-  % clang -emit-llvm -c a.c -o a.o   # <-- a.o is LLVM bitcode file
+  % clang -flto -c a.c -o a.o        # <-- a.o is LLVM bitcode file
   % clang -c main.c -o main.o        # <-- main.o is native object file
-  % clang a.o main.o -o main         # <-- standard link command without modifications
+  % clang -flto a.o main.o -o main   # <-- standard link command with -flto
 
 * In this example, the linker recognizes that ``foo2()`` is an externally
   visible symbol defined in LLVM bitcode file. The linker completes its usual
   symbol resolution pass and finds that ``foo2()`` is not used
   anywhere. This information is used by the LLVM optimizer and it
   removes ``foo2()``.
 
 * As soon as ``foo2()`` is removed, the optimizer recognizes that condition ``i