Fix plugins.md document and createPlugin type definition

[ybiquitous]

Which issue, if any, is this issue related to?

Closes #6245

Is there anything in the PR that needs further explanation?

This PR mainly fixes the incorrect examples on the plugin document. Also, this improves the type definition of the stylelint.createPlugin() API, although it may be called refactorings.

The incorrect part on the doc is here: ⬇️

stylelint/docs/developer-guide/plugins.md

Lines 53 to 55 in e424be5

	module.exports.ruleName = ruleName;
	module.exports.messages = messages;
	module.exports.meta = meta;

Because stylelint.createPlugin() returns an object { ruleName: string, rule: Function },

stylelint/lib/createPlugin.js

Lines 10 to 15 in 4772851

	function createPlugin(ruleName, rule) {
	return {
	ruleName,
	rule,
	};
	}

the following code using module.exports produces an incorrect object { ruleName: string, rule: Function, messages: Object, meta: Object }.

module.exports.ruleName = ruleName;
module.exports.messages = messages;
module.exports.meta = meta;

To be correct, the rule function should have the ruleName, messages, and meta properties as below. The current example on the doc is confusing.

const ruleFunc = () => { /* .. */ };

ruleFunc.ruleName = ruleName;
ruleFunc.messages = messages;
ruleFunc.meta = meta;

For example, the stylelint-scss plugin sets the properties correctly.

[jeddy3]

This all LGTM, thank you!

[ybiquitous]

@jeddy3 Thanks for the review!

Changelog entry added:

• Fixed: plugins document and createPlugin type definition (#6264).