List allowed values of gh config settings

[muzimuzhi]

Describe the feature or problem you’d like to solve

In addition to default values, having allowed values documented would be more helpful and comprehensive.
https://cli.github.com/manual/gh_config

Related to

• Enum values listed in option docs are poorly formatted #8349

Proposed solution

Document allowed values.

Additional context

cli/internal/config/config.go

Lines 544 to 579 in 541ce0e

	func ConfigOptions() []ConfigOption {
	return []ConfigOption{
	{
	Key: gitProtocolKey,
	Description: "the protocol to use for git clone and push operations",
	DefaultValue: "https",
	AllowedValues: []string{"https", "ssh"},
	},
	{
	Key: editorKey,
	Description: "the text editor program to use for authoring text",
	DefaultValue: "",
	},
	{
	Key: promptKey,
	Description: "toggle interactive prompting in the terminal",
	DefaultValue: "enabled",
	AllowedValues: []string{"enabled", "disabled"},
	},
	{
	Key: pagerKey,
	Description: "the terminal pager program to send standard output to",
	DefaultValue: "",
	},
	{
	Key: httpUnixSocketKey,
	Description: "the path to a Unix socket through which to make an HTTP connection",
	DefaultValue: "",
	},
	{
	Key: browserKey,
	Description: "the web browser to use for opening URLs",
	DefaultValue: "",
	},
	}
	}

[zsloane]

My changes in #8395 address this.

I would like to make it more generic and have the docs package handle it but for now this incremental improvement should do