Documentation: Add details for pstore/blk

Add details on using pstore/blk, the new backend of pstore to record
dumps to block devices, in Documentation/admin-guide/pstore-blk.rst

Signed-off-by: WeiXiong Liao <liaoweixiong@allwinnertech.com>
Link: https://lore.kernel.org/lkml/20200511233229.27745-7-keescook@chromium.org/


Signed-off-by: Kees Cook <keescook@chromium.org>