Refine help output for generic flags

[rm3l]

    Just a discussion (also applies to the `run-on` flag):

From the user perspective, now that these two flags are visible globally, can the odo help output provide more guidance by telling users which commands these flags can be used with? I understand this would mean maintaining these descriptions, but I think it would be helpful.
So I'd suggest something like:

	pflag.CommandLine.String(VarFileFlagName, "", "File containing variables to override Devfile variables. Usable with \"odo dev\" and \"odo deploy\" commands.")

What do you think?

Because currently, users have to explicitly run the command to see if the flag can be used or not, e.g.:

> odo init --var USER                                                             
 ✗  --var-file and --var flags are not supported for this command

Originally posted by @rm3l in #6201 (comment)

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[rm3l]

Still relevant from the CLI UX perspective.

/remove-lifecycle stale
/remove-lifecycle rotten
/priority Low

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A friendly reminder that this issue had no activity for 90 days. Stale issues will be closed after an additional 30 days of inactivity.