qemu-options: try and clarify preferred block semantics

Try to correct any confusion about QEMU's Byzantine disk options by
laying out the preferred "modern" options as-per:

 "<danpb> (best:  -device + -blockdev,  2nd obsolete syntax: -device +
     -drive,  3rd obsolete syntax: -drive, 4th obsolete syntax: -hdNN)"

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Cc: qemu-block@nongnu.org
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Hanna Reitz <hreitz@redhat.com>
Cc: Thomas Huth <thuth@redhat.com>
Message-Id: <20220822165608.2980552-7-alex.bennee@linaro.org>

qemu-options.hx

@@ -1105,6 +1105,19 @@ DEFHEADING()
 
 DEFHEADING(Block device options:)
 
+SRST
+The QEMU block device handling options have a long history and
+have gone through several iterations as the feature set and complexity
+of the block layer have grown. Many online guides to QEMU often
+reference older and deprecated options, which can lead to confusion.
+
+The recommended modern way to describe disks is to use a combination of
+``-device`` to specify the hardware device and ``-blockdev`` to
+describe the backend. The device defines what the guest sees and the
+backend describes how QEMU handles the data.
+
+ERST
+
 DEF("fda", HAS_ARG, QEMU_OPTION_fda,
     "-fda/-fdb file  use 'file' as floppy disk 0/1 image\n", QEMU_ARCH_ALL)
 DEF("fdb", HAS_ARG, QEMU_OPTION_fdb, "", QEMU_ARCH_ALL)