docs(api.md): mention header/footer template limitations in page.pdf.

[aslushnikov]

References #2167.

[RayAnti2]

I found puppeteer is a good tool for converting from html to pdf.
But the footerTemplate must be the same on all pages?
Now I need, for example, pages' footer like this:

(page 1)
pageNumber of totalNumber---------------------------vocabulary list

(page 2)
vocabulary list-------------------------------------pageNumber of totalNumber

(page 3)
pageNumber of totalNumber---------------------------vocabulary list

I prepare 2 Templates for odd and even num pages, i don't need every pages with different footers, but just these two Templates.
What should i do ?