Documentation: qemu-img: Mention SIGUSR1 progress report

Document the SIGUSR1 behaviour of qemu-img. Also, added compare to the
list of subcommands that support -p.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Reviewed-by: NBenoit Canet <benoit@irqsave.net>

qemu-img.texi

@@ -57,7 +57,9 @@ indicates that target image must be compressed (qcow format only)
 @item -h
 with or without a command shows help and lists the supported formats
 @item -p
-display progress bar (convert and rebase commands only)
+display progress bar (compare, convert and rebase commands only).
+If the @var{-p} option is not used for a command that supports it, the
+progress is reported when the process receives a @code{SIGUSR1} signal.
 @item -q
 Quiet mode - do not print any output (except errors). There's no progress bar
 in case both @var{-q} and @var{-p} options are used.