doc: small kernel-parameters updates

Change last "i386" to X86-32 as is used throughout the rest of the file.
Change combination of X86-32,X86-64 to just X86, as is done throughout the
rest of the file.

Add a note that hyphens and underscores are equivalent in parameter names,
with examples.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Jan Engelhardt <jengelh@medozas.de>
Cc: Christopher Sylvain <chris.sylvain@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>