docs/system: add a gentle prompt for the complexity to come

We all know the QEMU command line can become a fiendishly complex
beast. Lets gently prepare our user for the horrors to come by
referencing where other example command lines can be found in the
manual.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210305092328.31792-3-alex.bennee@linaro.org>

docs/system/quickstart.rst

@@ -11,3 +11,11 @@ Download and uncompress a PC hard disk image with Linux installed (e.g.
    |qemu_system| linux.img
 
 Linux should boot and give you a prompt.
+
+Users should be aware the above example elides a lot of the complexity
+of setting up a VM with x86_64 specific defaults and assumes the
+first non switch argument is a PC compatible disk image with a boot
+sector. For a non-x86 system where we emulate a broad range of machine
+types, the command lines are generally more explicit in defining the
+machine and boot behaviour. You will find more example command lines
+in the :ref:`system-targets-ref` section of the manual.

docs/system/targets.rst

+.. _system-targets-ref:
+
 QEMU System Emulator Targets
 ============================