qemu-options qemu-doc: Move "Device URL Syntax" to qemu-doc

qemu-options qemu-doc: Move "Device URL Syntax" to qemu-doc

Commit 0f5314a (v1.0) added section "Device URL Syntax" to
qemu-options.hx.  It's enclosed in STEXI..ETEXI, thus affects only
qemu-options.texi, not --help.  It appears as a subsection under
section "Invocation".  Similarly, qemu.1 has it as a subsection under
"OPTIONS".

Commit f9dadc9 (v1.1.0) dropped new option -iscsi into the middle of
this section.  No effect on qemu-options.texi.  It appears in --help
run together with the "Bluetooth(R) options:" header.

Commit c70a01e (v1.5.0) gives it is own heading in --help by moving
commit 0f5314a's DEFHEADING(Device URL Syntax:) outside STEXI..ETEXI.
Trouble is the heading makes no sense for -iscsi.

Move all of the "Device URL Syntax" Texinfo to qemu-doc.texi.  Mark it
for inclusion in qemu.1 with '@c man begin NOTES'.  This turns it into
a separate section outside the list of options both in qemu-doc and in
qemu.1.

There's substantial overlap with the existing qemu-doc section "Disk
Images".  Mark with a TODO comment.

Output of --help will be fixed next.

Cc: Ronnie Sahlberg <ronniesahlberg@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: Max Reitz <mreitz@redhat.com>
Cc: qemu-block@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20171002140307.5292-4-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>
[Unwanted @node dropped]