docs/BOOT_LOADER_INTERFACE: use full variable names once

docs/BOOT_LOADER_INTERFACE: use full variable names once

We said in the header that "all EFI variables use the vendor UUID
4a67b082-0a4c-41cf-b6c7-440b29bb8c4f", but people not familiar with
UEFI might not know that this is concatenated with the variable name.
Let's use the full form once — when introducing the variable — to
make it easier to grep and search for.

While at it, use sembreaks in the document. This makes subsequent
changes much easier to review. (It also shows that some sentences
are rather long and thus hard to understand.)