This may be a dup of bug #293455. The Optional: Framebuffer section in the handbook (Ch 10) details the video line that one may provide to the kernel, along with an admonition to "Write this setting down; you will need it shortly." I assume that most people coming to Gentoo now will use grub2 in order to have EFI support, but the framebuffer video line isn't mentioned at all in Sec. 10.b.
I think the framebuffer section is no longer present in the handbook (probably through bug 293455). There is a mention of a video= as an example on how to add kernel boot options though (https://wiki.gentoo.org/wiki/Handbook:AMD64/Installation/Bootloader), but that is just as an example and imo doesn't confuse people.