improve some types on phpdoc

[jamband]

fixed a mistake in the type of the argument of the encode method. also, there was an error in the return value type of the decode method, so I fixed it.

array{} means an empty array. this is fine without it, but it makes it explicit that an empty array can also be returned. you can remove it if you don't like it.

[vinkla]

Thanks for the pull request Tomoki!

Maybe should we drop the blocks on the class and rely just on the ones from the interface instead? Like we do with the math interface.

[jamband]

@vinkla yes, it makes the phpdoc block cleaner :)

[vinkla]

Great! Would you mind removing the blocks in this PR?

[jamband]

i pushed again because I included unnecessary commits. would this be ok?

[vinkla]

Looks good! Thanks 🙌

[szepeviktor]

This is a great PR.