Is your feature request related to a problem? Please describe.
Right now, if I describe a CommandSetting with [CommandOption("--option [RED|GREEN|BLUE]")], it fails with

Error: An error occured when parsing template.
       Encountered invalid character '|' in value name.

       --option [RED|GREEN|BLUE]
                    ^ Invalid character

Likely caused by this line:
https://github.com/spectreconsole/spectre.console/blob/main/src/Spectre.Console.Cli/Internal/Configuration/TemplateParser.cs#L91

Describe the solution you'd like
I don't really expect the validation to use the list I put in the help text, but I would like it to allow the concise help to be able to show a list of acceptable answers instead of having to put it in the description.

Describe alternatives you've considered
I tried commas, and with angle brackets.

I think my next best workaround is something like:

[CommandOption("--option <RED_or_GREEN_or_BLUE>")]

or

[CommandOption("--option <COLOR>")]
[Description("Options for COLOR are RED, GREEN, or BLUE.")]

But both of the above doesn't seem quite as concise as my proposed solution of supporting the pipe character.

Additional context
If there is a better way to do this for this library, maybe seeing an example in the docs would be nice.

Also, thanks for the awesome library. The docs are awesome, and I am loving the experience using it so far.

Please upvote 👍 this issue if you are interested in it.