Update "disassemble" help

Pedro pointed out that disassemble/m should be documented after
disassemble/s, because /m is deprecated.  This patch does so, and adds
a usage line.

Regression tested on x86-64 Fedora 32.

gdb/ChangeLog
2020-07-28  Tom Tromey  <tromey@adacore.com>

	* cli/cli-cmds.c (_initialize_cli_cmds): Rearrange "disassemble"
	help.  Add usage.