documentation for allocate_pages

[ofluffydev]

I was unable to tell what boot::allocate_pages returned without looking into the actual code since it's returning a u8, maybe a good spot to add more documentation? https://docs.rs/uefi/latest/uefi/boot/fn.allocate_pages.html

[phip1611]

Just done recently :) https://github.com/rust-osdev/uefi-rs/pull/1665/files

Therefore I'm closing this. Please reopen if you think we missed something