x86-64: Document some of entry_64.S

x86-64: Document some of entry_64.S

Signed-off-by: Andy Lutomirski <[email protected]>
Cc: Jesper Juhl <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Arjan van de Ven <[email protected]>
Cc: Jan Beulich <[email protected]>
Cc: richard -rw- weinberger <[email protected]>
Cc: Mikael Pettersson <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Louis Rilling <[email protected]>
Cc: [email protected]
Cc: [email protected]
Link: http://lkml.kernel.org/r/fc134867cc550977cc996866129e11a16ba0f9ea.1307292171.git.luto@mit.edu
Signed-off-by: Ingo Molnar <[email protected]>