Help message for kn service apply --env is appropriate

[rhuss]

Bug report

kn service apply --help yields:

-e, --env stringArray                   Environment variable to set. NAME=value; you may provide this flag any number of times to set multiple environment variables. To unset, specify the environment variable name followed by a "-" (e.g., NAME-).

This is not correct since you can't add or remove env vars with apply but just set the full set of env variables.

Same possibly for --labels and --annotations (or any map-based option)

Expected behavior

The help message for --env should be different for kn service update and kn service apply as the behaviour is different (update : incremental changes by adding/removing, apply: full state)

The same is true for kn service create where removing env vars also doesn't make any sense.

Thanks to @pmbanugo for reporting this issue.

[boaz0]

@rhuss can you assign this to me after I finish with issue 1232

[rhuss]

/assign boaz0