x86/msr: Document msr-index.h rule for addition

x86/msr: Document msr-index.h rule for addition

In order to keep this file's size sensible and not cause too much
unnecessary churn, make the rule explicit - similar to pci_ids.h - that
only MSRs which are used in multiple compilation units, should get added
to it.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: alex.williamson@redhat.com
Cc: gleb@kernel.org
Cc: joro@8bytes.org
Cc: kvm@vger.kernel.org
Cc: sherry.hurwitz@amd.com
Cc: wei@redhat.com
Link: http://lkml.kernel.org/r/1455612202-14414-5-git-send-email-bp@alien8.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>