docs: initramfs: update compression and mtime descriptions

Update the document to reflect that initramfs didn't replace initrd
following kernel 2.5.x.
The initramfs buffer format now supports many compression types in
addition to gzip, so include them in the grammar section.
c_mtime use is dependent on CONFIG_INITRAMFS_PRESERVE_MTIME.

Signed-off-by: David Disseldorp <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Randy Dunlap <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

Documentation/driver-api/early-userspace/buffer-format.rst

···
 
 Al Viro, H. Peter Anvin
 
-Last revision: 2002-01-13
-
-Starting with kernel 2.5.x, the old "initial ramdisk" protocol is
-getting {replaced/complemented} with the new "initial ramfs"
-(initramfs) protocol.  The initramfs contents is passed using the same
-memory buffer protocol used by the initrd protocol, but the contents
+With kernel 2.5.x, the old "initial ramdisk" protocol was complemented
+with an "initial ramfs" protocol.  The initramfs content is passed
+using the same memory buffer protocol used by initrd, but the content
 is different.  The initramfs buffer contains an archive which is
-expanded into a ramfs filesystem; this document details the format of
-the initramfs buffer format.
+expanded into a ramfs filesystem; this document details the initramfs
+buffer format.
 
 The initramfs buffer format is based around the "newc" or "crc" CPIO
 formats, and can be created with the cpio(1) utility.  The cpio
-archive can be compressed using gzip(1).  One valid version of an
-initramfs buffer is thus a single .cpio.gz file.
+archive can be compressed using gzip(1), or any other algorithm provided
+via CONFIG_DECOMPRESS_*.  One valid version of an initramfs buffer is
+thus a single .cpio.gz file.
 
 The full format of the initramfs buffer is defined by the following
 grammar, where::
···
 	*	is used to indicate "0 or more occurrences of"
 	(|)	indicates alternatives
 	+	indicates concatenation
-	GZIP()	indicates the gzip(1) of the operand
+	GZIP()	indicates gzip compression of the operand
+	BZIP2()	indicates bzip2 compression of the operand
+	LZMA()	indicates lzma compression of the operand
+	XZ()	indicates xz compression of the operand
+	LZO()	indicates lzo compression of the operand
+	LZ4()	indicates lz4 compression of the operand
+	ZSTD()	indicates zstd compression of the operand
 	ALGN(n)	means padding with null bytes to an n-byte boundary
 
-	initramfs  := ("\0" | cpio_archive | cpio_gzip_archive)*
+	initramfs := ("\0" | cpio_archive | cpio_compressed_archive)*
 
-	cpio_gzip_archive := GZIP(cpio_archive)
+	cpio_compressed_archive := (GZIP(cpio_archive) | BZIP2(cpio_archive)
+		| LZMA(cpio_archive) | XZ(cpio_archive) | LZO(cpio_archive)
+		| LZ4(cpio_archive) | ZSTD(cpio_archive))
 
 	cpio_archive := cpio_file* + (<nothing> | cpio_trailer)
 
···
 
 The c_mode field matches the contents of st_mode returned by stat(2)
 on Linux, and encodes the file type and file permissions.
+
+c_mtime is ignored unless CONFIG_INITRAMFS_PRESERVE_MTIME=y is set.
 
 The c_filesize should be zero for any file which is not a regular file
 or symlink.