Add documentation for ROP magic register properties and setRegisters

[heapcrash]

No description provided.

[heapcrash]

This also needs to document this neat way of setting registers that I just learned from another issue. Apparently you can do:

    >>> context.clear(arch='amd64')
    >>> assembly = 'pop rax; pop rdi; pop rsi; ret; pop rax; ret;'
    >>> e = ELF.from_assembly(assembly)
    >>> r = ROP(e)
    >>> r(rax=0xdead, rdi=0xbeef, rsi=0xcafe) <----------------- MAGIC
    >>> print(r.dump())
    0x0000:       0x10000000 pop rax; pop rdi; pop rsi; ret
    0x0008:           0xdead
    0x0010:           0xbeef
    0x0018:           0xcafe

[heapcrash]

This is a documentation change, so no CHANGELOG.md is necessary.

[Arusekk]

On the second thought we could also document the setattr magics as well (r.rax = 0xdead is equivalent to r(rax=0xdead)).