docs: tweaks to the boot loader spec

docs: tweaks to the boot loader spec

A couple of changes:

1. Clearly name the drop-in entry files "Type #1", and the unified
   kernel images "Type #2", and be clearer that the latter is specific
   to UEFI.

2. Suffix all directory paths with a trailing "/" to clarify that these
   are directories. Also, enclose them all in ``.

3. Add introductory paragraph that explains that there is Type #1 and
   Type #2 and what they are about.

4. Explain that Type #2 is about signed UEFI SecureBoot.

5. Don't claim that $BOOT/loader/ contains really all files defined by
   the spec, because that's not true, Type #2 images are not located there
   after all.

Fixes: #10399