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.

docs/BOOT_LOADER_INTERFACE.md

@@ -74,3 +74,41 @@ that directory is empty, and only if no other file systems are mounted
 there. The `systemctl reboot --boot-loader-entry=…` and `systemctl reboot
 --boot-loader-menu=…` commands rely on the `LoaderFeatures` ,
 `LoaderConfigTimeoutOneShot`, `LoaderEntries`, `LoaderEntryOneShot` variables.
+
+## Boot Loader Entry Identifiers
+
+While boot loader entries may be named relatively freely, it's highly
+recommended to follow the following rules when picking identifiers for the
+entries, so that programs (and users) can derive basic context and meaning from
+the identifiers as passed in `LoaderEntries`, `LoaderEntryDefault`,
+`LoaderEntryOneShot`, `LoaderEntrySelected`, and possibly show nicely localized
+names for them in UIs.
+
+1. When boot loader entries are defined through [Boot Loader
+   Specification](https://systemd.io/BOOT_LOADER_SPECIFICATION) drop-in files
+   the identifier should be derived directly from the drop-in snippet name, but
+   with the `.conf` (or `.efi` in case of Type #2 entries) suffix removed.
+
+2. Entries automatically discovered by the boot loader (as opposed to being
+   configured in configuration files) should generally have an identifier
+   prefixed with `auto-`.
+
+3. Boot menu entries referring to Microsoft Windows installations should either
+   use the identifier `windows` or use the `windows-` prefix for the
+   identifier. If a menu entry is automatically discovered, it should be
+   prefixed with `auto-`, see above (Example: this means an automatically
+   discovered Windows installation might have the identifier `auto-windows` or
+   `auto-windows-10` or so.).
+
+4. Similar, boot menu entries referring to Apple MacOS X installations should
+   use the identifier `osx` or one that is prefixed with `osx-`. If such an
+   entry is automatically discovered by the boot loader use `auto-osx` as
+   identifier, or `auto-osx-` as prefix for the identifier, see above.
+
+5. If a boot menu entry encapsulates the EFI shell program, it should use the
+   identifier `efi-shell` (or when automatically discovered: `auto-efi-shell`,
+   see above).
+
+6. If a boot menu entry encapsulates a reboot into EFI firmware setup feature,
+   it should use the identifier `reboot-to-firmware-setup` (or
+   `auto-reboot-to-firmware-setup` in case it is automatically discovered).