Think about it, you can write a poem with the secret words involved, or put them involved with 1000 unrelated words or in unreleated Ebook and even miss word or two and remember them
Don't worry I have all seeds saved in email in form of a story and some highlights are done on those keywords.
Actually they do not advice you to do so because if someone compromises your email account or the email provider then he will get all the details in no time. But I think writing on a paper is more vulnerable to theft, losing, and getting torn/damaged!
I do not worry about it because I have 4 counts of security measures.
1. I am using Gmail and nobody will ever be able to compromise Google.
2. I have wrote them in form of a short story which will be hard to figure out except me.
3. I have encrypted the whole story with my PGP key and saved there.
4. I have 2FA enabled with my mobile which I do not use outside my house.
Really good plan... Did you hear about how easy dropbox employees had access to peoples files that were stored there? The same is probably true for your e-mail.
Also, just saying this may put a target on your account. Hacks can always happen, social engineering may even be your biggest enemy here.
You do what you got to do, but I wouldn't advise this. You probably don't need to log into your gmail at your home pc or laptop right? If someone steals that, they have acces to your key. And you probably didn't "hide" it sufficiently once this happens.