docs(luacats): prevent lua-ls `redundant-return-value` warning

[mrcjkb]

Hey :wave:

we run lua-language-server to statically type check our code in CI. It fails on warnings when we pass in a function that returns something to nio.create.

This is because the luaCATS annotation specifies ---@param fun(...) (without a return value). Specifying function instead should prevent this.

[rcarriga]

Thanks for the PR!