net: lwm2m: Clean up shell documentation

Move commands that require parameters first into the list.

Move syntax line first, command documentation second,
flags last. This is much like Unix commands do

❯ /bin/ls --help
Usage: /bin/ls [OPTION]... [FILE]...
List information about the FILEs (the current directory by default).

Compared to:

uart:~$ lwm2m
lwm2m - LwM2M commands
Subcommands:
  read    :read PATH [OPTIONS]
          Read value from LwM2M resource
          -x   Read value as hex stream (default)
          -s   Read value as string

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>