Make readings list help msg more informative

[difince]

🐞 Bug Report

Affected Command

./edgex-cli reading list

Is this a regression?

No

Description and Minimal Reproduction

• Help message is not enough informative. It says "Return all device readings." All readings belong to device - so it is not clear if the help msg meant - returning all readings or returning all reading that belongs to specific device. It does not specify if device id or device name should be provided. There is mismatch in edgex-go between documentation and implementation - see issue mismatch between documentation and implementation of delete endpoints /event/device/{deviceId} and reading/device/{deviceId}/{limit} edgex-go#2591. There is no flag for specifying the device, so the device need to be sent as an Argument but it is somehow not clear.
  In addition the --limit flag is applied only if a device arg is provided. Otherwise it is not taken in consideration. (it should be obviously said). go-mod-contracts does not have a function that returns all events limited by number. @mkbhanda have opened an issue about that in edgex-go No limit on /v1/reading edgex-go#2560

If device argument is not provided, the current behavior is to list all readings if they are not too much.
If the readings are too much an error is returned

Error: 500 - limit 69584 exceeds configured max
null

So, an usage example and additional information in the help message should be provided.

./edgex-cli reading list -h

Return all device readings.

Usage:
edgex-cli reading list [flags]

Flags:
-h, --help help for list
-l, --limit int32 Limit number of results

Global Flags:
--config-file string configuration file
--no-pager Do not pipe output into a pager.
-u, --url Print URL(s) used by the entered command.
-v, --verbose Print entire HTTP response.

Here is a working example:
./edgex-cli reading list Random-Integer-Device -l 2