I have designed a website for one of my client's and he is just asking for some documentation, my question here is Am I responsible for documenting the website or it is the web developer's or SEO work, please guide me.

I have designed a website for one of my client's and this is my first freelance work in which I have designed the complete front-end look of the website, he is just asking for some documentation, I don't know what this term actually means, my question here is Am I really responsible for documenting the website, which means is that mandatory or optional, please guide me.