doc: document the boot menu entry identifier vocabulary

The existing text already said "See below regarding a recommended
vocabulary for boot loader entry identifiers.", but the section for it
was still missing. Let's fill in the missing bits, and describe basic
suggested rules for the boot menu entry identifier vocabulary, in
particular how to identify Windows and MacOS X installations, and how to
name automatic entries vs. explicitly configured ones.

This basically follows the logic implemented in sd-boot these days.