document set_segment_register and related function
document dpl/rpl references
Document TR/descriptor type references
document lsl/lar boolean
document 32/16bit var
document lar/lsl descriptor checking
document selector var
Change comment to match naming convention in Intel manual
document stack accesses in do_return_paged_mode and do_return_not_paged
document immediate field in RETF
document stack pointer usage
document paged mode return stack references
Merge pull request #1 from danluu/master

document some call, ret, jmp, etc. relation functions and vars