xz: Documentation/staging/xz.rst: Revise thoroughly

xz: Documentation/staging/xz.rst: Revise thoroughly

Add SPDX license identifier.

Omit xz_dec_test info. That isn't relevant to developers of non-XZ code.

Revise the docs about xzkern and add xzkern_with_size.  The latter was
added to scripts/Makefile.lib in the commit 7ce7e984ab2b ("kbuild: rename
cmd_{bzip2,lzma,lzo,lz4,xzkern,zstd22}").

Omit contact info as MAINTAINERS has it.

Omit other info that is outdated or not relevant in the kernel context.

Include the xz_dec kernel-doc from include/linux/xz.h.

Link: https://lkml.kernel.org/r/20240721133633.47721-8-lasse.collin@tukaani.org
Signed-off-by: Lasse Collin <lasse.collin@tukaani.org>
Reviewed-by: Sam James <sam@gentoo.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Rui Li <me@lirui.org>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Emil Renner Berthing <emil.renner.berthing@canonical.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: Joel Stanley <joel@jms.id.au>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Jubin Zhong <zhongjubin@huawei.com>
Cc: Jules Maselbas <jmaselbas@zdiv.net>
Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>