Clarify how llvm-mca detects att vs intel syntax.

[andreadb]

LGTM

Thanks!

[jlebar]

Thank you for the review, and I'm sorry for falling off the face of the earth for a few weeks. Somehow I misconfigured either phab or gmail so I wasn't receiving any review emails!

I'll submit this once I get commit access to github.

llvm/docs/CommandGuide/llvm-mca.rst

.. code-block:: bash
   $ clang foo.c -O2 -target x86_64-unknown-unknown -S -o - | llvm-mca -mcpu=btver2
 
 Or for Intel syntax:
 
 .. code-block:: bash
 
   $ clang foo.c -O2 -target x86_64-unknown-unknown -mllvm -x86-asm-syntax=intel -S -o - | llvm-mca -mcpu=btver2
 
+(:program:`llvm-mca` detects Intel syntax by the presence of an `.intel_syntax`
+directive at the beginning of the input.  By default its output syntax matches
+that of its input.)
+
 Scheduling models are not just used to compute instruction latencies and
 throughput, but also to understand what processor resources are available
 and how to simulate them.
 
 By design, the quality of the analysis conducted by :program:`llvm-mca` is
 inevitably affected by the quality of the scheduling models in LLVM.
 
 If you see that the performance report is not accurate for a processor,