docs: Added cutoff values for scale option in page.pdf function

[jsfeldman]

I thought it would be useful to add the lowest and highest values for the scale option to the page.pdf function call in the api documentation. The library throws an error and doesn't generate a pdf if you go beyond those values. I didn't manually test it, the values are based on the warning Chrome gives you if you try to set the scale value outside of that range (divided by 100), so let me know if they're wrong.

It may be useful to change the behavior in the future so that if the value is set beyond that range puppeteer sets the value to the closest value within the range and throws a warning.

Let me know what you think!

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here (e.g. I signed it!) and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

[jsfeldman]

I signed it!

[googlebot]

CLAs look good, thanks!

[aslushnikov]

Thanks!