efi/metadata: Make a note that efi_map_header isn't standard

efi_map_header is similar to, but not at all the same as the UEFI
EFI_MEMORY_ATTRIBUTES_TABLE (we could easily have used the latter
though, with one fewer non-standard types, but we can't change
it easily now due to the last 10 years of boot loaders passing
this in).

Sponsored by:		Netflix

sys/arm/include/metadata.h

@@ -33,10 +33,15 @@
 #define	MODINFOMD_DTBP		0x1002
 #define	MODINFOMD_EFI_MAP	0x1003
 
+/*
+ * This is not the same as the UEFI standard EFI_MEMORY_ATTRIBUTES_TABLE, though
+ * memory_size / descritpr_size entries of EFI_MEMORY_DESCRIPTORS follow this table
+ * starting at a 16-byte alignment.
+ */
 struct efi_map_header {
-	uint64_t	memory_size;
-	uint64_t	descriptor_size;
-	uint32_t	descriptor_version;
+	uint64_t	memory_size;		/* Numnber of bytes that follow */
+	uint64_t	descriptor_size;	/* Size of each EFI_MEMORY_DESCRIPTOR */
+	uint32_t	descriptor_version;	/* Currently '1' */
 };
 
 /*

sys/arm64/include/metadata.h

@@ -31,10 +31,15 @@
 #define	MODINFOMD_DTBP		0x1002
 #define	MODINFOMD_EFI_FB	0x1003
 
+/*
+ * This is not the same as the UEFI standard EFI_MEMORY_ATTRIBUTES_TABLE, though
+ * memory_size / descritpr_size entries of EFI_MEMORY_DESCRIPTORS follow this table
+ * starting at a 16-byte alignment.
+ */
 struct efi_map_header {
-	size_t		memory_size;
-	size_t		descriptor_size;
-	uint32_t	descriptor_version;
+	size_t		memory_size;		/* Numnber of bytes that follow */
+	size_t		descriptor_size;	/* Size of each EFI_MEMORY_DESCRIPTOR */
+	uint32_t	descriptor_version;	/* Currently '1' */
 };
 
 struct efi_fb {

sys/riscv/include/metadata.h

@@ -33,10 +33,15 @@
 #define	MODINFOMD_EFI_FB	0x1003
 #define	MODINFOMD_BOOT_HARTID	0x1004
 
+/*
+ * This is not the same as the UEFI standard EFI_MEMORY_ATTRIBUTES_TABLE, though
+ * memory_size / descritpr_size entries of EFI_MEMORY_DESCRIPTORS follow this table
+ * starting at a 16-byte alignment.
+ */
 struct efi_map_header {
-	size_t		memory_size;
-	size_t		descriptor_size;
-	uint32_t	descriptor_version;
+	size_t		memory_size;		/* Numnber of bytes that follow */
+	size_t		descriptor_size;	/* Size of each EFI_MEMORY_DESCRIPTOR */
+	uint32_t	descriptor_version;	/* Currently '1' */
 };
 
 /*

sys/x86/include/metadata.h

@@ -36,10 +36,15 @@
 #define	MODINFOMD_VBE_FB	0x1007
 #define	MODINFOMD_EFI_ARCH	0x1008
 
+/*
+ * This is not the same as the UEFI standard EFI_MEMORY_ATTRIBUTES_TABLE, though
+ * memory_size / descritpr_size entries of EFI_MEMORY_DESCRIPTORS follow this table
+ * starting at a 16-byte alignment.
+ */
 struct efi_map_header {
-	uint64_t	memory_size;
-	uint64_t	descriptor_size;
-	uint32_t	descriptor_version;
+	uint64_t	memory_size;		/* Numnber of bytes that follow */
+	uint64_t	descriptor_size;	/* Size of each EFI_MEMORY_DESCRIPTOR */
+	uint32_t	descriptor_version;	/* Currently '1' */
 };
 
 struct efi_fb {