Clarify description for cf restart-app-instance command

[tcdowney]

What's the user value of this feature request?
The current description for the restart-app-instance command is confusing. Unlike the regular cf restart command, this command does not update the app environment (environment variables, service bindings, app renaming, etc.) for the instance.

Current help text:

cf restart-app-instance -h
NAME:
   restart-app-instance - Terminate, then instantiate an app instance

USAGE:
   cf restart-app-instance APP_NAME INDEX [--process PROCESS]

OPTIONS:
   --process      Process to restart (Default: web)

Who is the functionality for?
App developers / app operators.

How often will this functionality be used by the user?
Whenever they wish to learn more about cf restart-app-instance.

Who else is affected by the change?
No one, it merely clarifies existing behavior.

Is your feature request related to a problem? Please describe.
Users run cf restart-app-instance and expect it to behave like cf restart. When it doesn't they are surprised and confused.

Describe the solution you'd like
Update the description to the command to something like:

Stop, then start application instance without updating application environment

(thanks @Benjamintf1 for talking through suggestions with me)

Example new help text:

cf restart-app-instance -h
NAME:
   restart-app-instance - Stop, then start application instance without updating application environment

USAGE:
   cf restart-app-instance APP_NAME INDEX [--process PROCESS]

OPTIONS:
   --process      Process to restart (Default: web)

Additional context
Under the hood Cloud Controller calls the RetireActualLRP endpoint on BBS.

// Shuts down the ActualLRP matching the given ActualLRPKey, but does not modify the desired state
	RetireActualLRP(logger lager.Logger, traceID [string](https://pkg.go.dev/builtin#string), key *[models](https://pkg.go.dev/code.cloudfoundry.org/bbs/models).[ActualLRPKey](https://pkg.go.dev/code.cloudfoundry.org/bbs/models#ActualLRPKey)) [error](https://pkg.go.dev/builtin#error)

[gururajsh]

Hi @tcdowney ,

I have added this issue to our tracker and will let you know when it's complete.

[gururajsh]

PRs are in. This change should be available with next release.