nasmdoc: corrections on 64-bit immediates/displacements
nasmdoc: corrections on 64-bit immediates/displacements

Corrections the section on 64-bit immediates and displacements.  In
particular, immediates are sign-extended the same way displacements
are (and the same way 8-bit immediates are), so there is some use for
the 7-byte mov eax,dword imm form :(

turingstudio/nasm,Distrotech/nasm,techkey/nasm,turingstudio/nasm,techkey/nasm,turingstudio/nasm,techkey/nasm,projedi/nasm,letolabs/nasm,projedi/nasm,projedi/nasm,Distrotech/nasm,turingstudio/nasm,techkey/nasm,Distrotech/nasm,projedi/nasm,letolabs/nasm,projedi/nasm,techkey/nasm,Distrotech/nasm,letolabs/nasm