Added a comment about pluralRules to the code in pluralization docs

[M1h4n1k]

I removed meaningless comment and replaced it with more useful comment about pluralRules for Composition api.
If the code is understandable I am just copying it and I'm not really reading through the documentation, and seems like many people do it the same way. However, in the docs there is a very important note. So, there is a higher chance that people will notice it if it's put directly into the code.

I've actually done the same mistake in 2 different projects. Luckily, at the second time I remembered that there was something wrong with docs and it didn't take me very long, but at the first time I spent quite a lot of time