Documentation/acpi/einj: Correct and streamline text

Streamline and simplify formulations, improve formatting and extend the
injection example in the error injection write up for users which we
carry in Documentation/.

Add a paragraph about checking for EINJ support and expand the ACPI5.0
memory errors section, as requested by Tony.

Acked-by: Tony Luck <tony.luck@intel.com>
Link: http://lkml.kernel.org/r/1422553845-30717-1-git-send-email-bp@alien8.de
Signed-off-by: Borislav Petkov <bp@suse.de>