[llvm-mca][docs] clarify how the quality of the perf report is affected by the quality of the scheduling models.

[andreadb]

Summary

The title says all...

Let me know if okay to commit.

[andreadb]

Forgot to add llvm-commits to the subscribers. Apologies for the spam.

[lebedev.ri]

Makes sense to me at least. Should this be cross-referenced somewhere else?

[mattd]

docs/CommandGuide/llvm-mca.rst

46

This reads nicely, but I suggest adding "scheduling" to be clearer about what models. "[...] is inevitably affected by the quality of the scheduling models in LLVM."

[andreadb]

Thanks for the reviews @lebedev.ri and @mattd

In D63556#1550415, @lebedev.ri wrote:

Makes sense to me at least. Should this be cross-referenced somewhere else?

My opinion is that this doesn't need to be cross-referenced.

docs/CommandGuide/llvm-mca.rst

46

I will change it. Thanks

[RKSimon]

In D63556#1550415, @lebedev.ri wrote:

Makes sense to me at least. Should this be cross-referenced somewhere else?

We'd need https://bugs.llvm.org/show_bug.cgi?id=37516 for that

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
 
+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 models in LLVM.
+
+If you see that the performance report is not accurate for a processor,
+please `file a bug <https://bugs.llvm.org/enter_bug.cgi?product=libraries>`_
+against the appropriate backend.
+
 OPTIONS
 -------
 
 If ``input`` is "``-``" or omitted, :program:`llvm-mca` reads from standard
 input. Otherwise, it will read from the specified filename.
 
 If the :option:`-o` option is omitted, then :program:`llvm-mca` will send its output
 to standard output if the input is from standard input.  If the :option:`-o`